Additional current work does not rewrite historical schedules. Have the actual qualifying period assessed instead of treating extra pay as a way to convert the past.
Budget for accurate records and a realistic timeline
Separate business compensation decisions from professional review, translations and current official charges. A backdated full-time description would not be an acceptable substitute for truthful evidence. Ask payroll or human resources for the actual dated timesheets, contracts or pay records from the period in question, rather than adjusting current compensation in a way that might be read as implying a different arrangement existed in the past.
Where a genuine inconsistency exists between an old record and what actually happened, request a written, dated explanation that reconciles the two rather than simply replacing the earlier document. Keep any decision about a current raise or bonus entirely separate from the historical accounting question, since combining them can make an honest current change look like an attempt to alter the past. Price the review of old records separately from any current payroll decision so the two are never presented as one transaction.
Ask whether a signed acknowledgment from a supervisor at the time, even if brief, already exists somewhere in old correspondence, since a short contemporaneous note is more persuasive than any amount of newly written explanation. Price the search for that kind of original material ahead of any decision about rewriting compensation records, since the search alone often resolves the question at a fraction of the cost.