So using NPP since you referenced it already...
NPP has a half life 3 to 4 days, steady state is usually reached in 2 weeks. For Deca half life 6 to 12 days depending, steady state takes 4 to 6 weeks. So if you switch the short ester clears. The long ester gradually builds. There will be an overlap period.
You don’t carry saturation over. You’re just swapping esters of the same base hormone.
To be effective you would want to maintain the short ester, while introducing the long. What I'm describing is essentially a crossover period, not a hard switch.
When moving from Nandrolone phenylpropionate (NPP) to, Nandrolone decanoate (Deca), you don’t stop NPP the same day you inject Deca because NPP clears relatively fast (short ester), Deca takes several weeks to build to steady levels.
If you stopped NPP immediately, you'd create a nandrolone dip while Deca is still climbing.
This is irrelevant if your testing tolerance, and in a follow up cycle you utilize the other. But in the case, where your trying to switch, it will be imperative to introduce simultaneously. This dosing will rely on actual milligrams being used.
Hope that helps.