Legacy Modernisation Strategies — UK Decision Framework
| Strategy | What It Means | Best For | Typical Cost | Timeline |
|---|---|---|---|---|
| Rehost (Lift and Shift) | Move to cloud without code changes | AWS cost saving, Cyber Essentials OS patching relief | £8,000–£25,000 | 4–8 weeks |
| Replatform | Minimal code changes — managed DB, containerise | Reduce patching burden, improve reliability | £15,000–£50,000 | 8–16 weeks |
| FHIR Facade | Wrap legacy in FHIR R4 API without replacing | NHS interoperability mandate — fastest path | £18,000–£40,000 | 12–18 weeks |
| Strangler Fig | Incrementally replace — new system runs alongside legacy | Zero business disruption, validated migration | £40,000–£150,000 | 24–52 weeks |
| Full Rewrite | Replace entirely — new architecture from scratch | Legacy beyond repair, total cost of ownership | £80,000–£200,000 | 36–60 weeks |
UK Regulated Sector Legacy Modernisation — Additional Requirements
| Sector | Pre-Modernisation Requirement | Modernisation Constraint | Post-Migration Validation |
|---|---|---|---|
| NHS / MedTech | IEC 62304 retrospective documentation, DCB0129 legacy hazard assessment | FHIR R4 must be maintained throughout migration (no dark periods) | Clinical data migration audit (5% sample clinical review) |
| FCA FinTech | PS21/3 IBS mapping (which legacy components support IBSs) | IBSs must remain within Impact Tolerance during migration | Post-migration IBS availability verification (90 days monitoring) |
| GDS Government | GDS service standard re-assessment (if significantly changed) | GOV.UK Pay/Notify integrations must be maintained | GDS Beta assessment for significantly changed services |
| InsurTech | Lloyd's CDR compatibility assessment (legacy CDR vs new) | Lloyd's CDR submissions must continue without gap | Lloyd's Risk Exchange parallel validation (4 weeks) |
| Pension / COLL | HMRC RTI continuity (pension payment reporting must not lapse) | PDP (Pension Dashboards) APIs maintained throughout | HMRC RTI reconciliation (first 3 submissions after go-live) |
The Technical Discovery — Why It Is Non-Negotiable
ClickMasters rule: no legacy modernisation contract without Technical Discovery. If a supplier offers a legacy modernisation fixed-price quote without a Discovery phase, they have not understood your system. The Discovery (£3,500–£8,000) always pays for itself — typically by preventing £20,000–£100,000 of rework.