Well they are going to fluctuate at least SOME. How do the doses compare?
If I was trying to keep levels approximately the same I would wait a week or ten days from my last cyp injection and then start the prop.
The first 2 weeks I'd just add whatever cyp dose, then week 3 drop the prop dose down a bit, then down even more, and come week 5 no prop just the cyp.
The prop will clear quickly, but it'll take 3-4 weeks to feel the cypionate.