We delivered the rescue and upgrade across four phases over approximately two months, following the same rescue framework we apply to every inherited Magento store.

Phase 1 — Audit, Stabilization & Infrastructure Migration

Stage 1 of any rescue is "stop the bleeding." For CNM, that meant getting the immediate operational risks under control before touching the upgrade work.

Work in this phase:

• Full technical audit — Magento 2.3.5 version state, PHP 7.3 runtime, server configuration, log analysis, indexer status, cron health, full module inventory

• Infrastructure migration to new dedicated server — the existing hosting wasn't going to support the upgraded stack the business needed. We provisioned a new UK-based dedicated server sized for the catalog and traffic profile, with the headroom to handle peak B2B procurement volume without throttling

• LiteSpeed Web Server setup with LSCache — chosen over Varnish for this engagement, LSCache provides full-page caching with native Magento 2 integration and significantly less configuration overhead than the standard Varnish stack. LiteSpeed also handles HTTP/3 cleanly, which UK Core Web Vitals measurement increasingly rewards

• Stabilization checklist — production mode confirmed, JS/CSS merging and minification, image conversion to WebP, lazy loading enabled across the catalog

• Security baseline — 2FA for all admin accounts, automated backups configured and verified, Content Security Policy applied

• Immediate bug triage — the most disruptive payment and checkout bugs addressed before the larger upgrade was scheduled

After Phase 1, the store stopped degrading and stabilized on infrastructure ready to absorb the upgrade work.

Phase 2 — Magento 2.3.5 → 2.4.8 Staircase Upgrade & PHP Migration

The Magento upgrade ran through Adobe's recommended upgrade path — not a direct jump, which isn't supported across five minor versions. The staircase:

2.3.5 → 2.3.7-p3 (final 2.3 patch, mandatory pre-2.4 step)

2.3.7 → 2.4.0 → 2.4.3 → 2.4.6 → 2.4.8 (sequential through 2.4.x versions, with PHP and database compatibility checks at each step)

Critical technical transitions in this phase:

• PHP 7.3 → PHP 8.x — runtime migration with full compatibility validation against every module, theme, and custom integration in the codebase

• ElasticSearch → OpenSearch 2.19+ — mandatory as of Magento 2.4.8 (April 2025 release). Catalog index rebuilt; faceted filtering validated across all category trees including the bathroom/heating product hierarchy

• Composer dependency resolution — third-party module compatibility validated at each minor version

• Database schema migrations — applied through each staircase step with full backup verification before progression

• Theme and frontend updates — visual refinements during the upgrade window without a full redesign, keeping the brand experience consistent for returning customers while modernizing the underlying frontend stack

Each staircase step ran first on staging with regression testing against production data before proceeding. Most of the engineering hours in Phase 2 went into module-by-module validation — a normal Magento store has 20-40 third-party modules; CNM's catalog and B2B/B2C dual operation involved enough integrations that every step needed careful validation.

Phase 3 — Payment Integration Fixes & B2B Invoice Workflow

This was the phase that addressed the operational pain points the business felt every day. CNM operates a six-method payment stack designed for the way UK public-sector and consumer customers actually pay:

• Credit/Debit Card (American Express, Visa, MasterCard, Discover)

• PayPal Express Checkout — fastest consumer path

• Super – Pay by Bank App — UK Open Banking payment with customer cashback rewards (a modern Open Banking option growing in UK retail)

• Proforma Invoice / Bank Transfer — preferred for B2B accounts requiring advance documentation

• Purchase Order — required by NHS trusts, councils, and universities operating under procurement frameworks

• Phone Order — legacy and accessibility option, important for older buyers and renovators preferring direct contact

Every integration was audited, fixed where buggy, and validated end-to-end on the new 2.4.8 stack. The Purchase Order and Proforma workflows received the most attention — public-sector procurement has specific approval, documentation, and delayed-billing requirements that consumer checkout doesn't.

Custom B2B invoice workflow:

• Custom PDF invoice templates built to meet UK B2B procurement expectations — proper line items, VAT breakdown, supplier registration details, and the specific layout NHS/council/university accounts teams require

• VAT logic validated for UK rates with B2B handling where applicable

• Quote-to-Invoice workflow — customer requests quote, sales team reviews and sends formal quote, customer approves, invoice generated against approved quote. This is what makes B2B sales workflow actually function vs. just having a "Request Quote" button on the site

• Email templates for transactional flows — quote sent, quote approved, invoice issued, payment received — rebuilt with consistent branding and the correct UK contact and VAT details

Performance optimization ran alongside:

• LiteSpeed LSCache configuration tuned for the catalog hit pattern

• Image pipeline optimization across the catalog

• Database query optimization for the cart and checkout paths

• Critical CSS inlining for above-the-fold rendering

Phase 4 — Launch, Monitoring & Ongoing

Launch shipped the upgraded CNM platform on Magento 2.4.8 with PHP 8, LiteSpeed infrastructure, fixed payment integrations, and the B2B invoice workflow operational. Post-launch work continues as an active partnership: ongoing development, performance monitoring, quarterly audits, and the targeted improvements an active UK supplier serving NHS-scale customers needs to stay compliant and competitive.