The narrative in Sg is there this may be a massive mixed buy of both A and B models - similar to Japan. Technically, RSAF has not replaced its 49-50 F5 S/Ts which the F35As could end up replacing.
The 40 to 60+ F15SGs replaced some or all of the Super Skyhawks. The F35Bs will replace the 70+ F16Vs as planned.

RSAF fighter replacements

Super Skyhawks -> replaced by 4.5+ gen F15SGs -> to be upgraded (midlife) and then replaced by a 6th gen fighter post 2040
F16s C/D/D+ -> upgraded (midlife) to F16V standard -> replaced by F35B
F5 S/T -> ? (likely F35As)