I always use used slot(s), replace with new keys so old fob never work again.
never have problems.
And also did that with keyless MB key, tried in all other slots, 3,4,..8,
no problems at all.
Renew or just program key without renew, it doesn't matter.

If there is start or sync problem,
just use diagnosis tool to check how key code roll, fix is easy.
If you only deal with key tool, you can fix any problems.