Officially Sage says at least 4.5 PU 8 but I think you can go 2017 and then 2019.
I asked Sage this question a few weeks ago and they described the reasoning behind this as:
The reasoning behind the decision to require users to be at 4.50 PU8 to convert to 2018 (6.0) are below:
All modules except P/R, J/C, W/O and MRp would have previously been converted to framework levels and the bulk of the extensive conversions would have been completed.
The conversion checks the version, not installed modules and we could not change this requirement based on which modules (P/R, J/C) were or were not installed.
4.50 PU8 contained the ACA (Affordable Care Act) and IRD compliance schema and code which minimized the conversion to 2018 Payroll.
The team had the highest confidence in the level conversion from 4.50.8 to 2018.
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Wayne Schulz - Schulz Consulting - 860-516-8990
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