ok.. From what i understand you want to take your established HOB filter out and replace it with this new one.
Danger and concern is how long until the Canister filter can handle the load the HOB is presently taking.
a few things...
Your filter does not hold your entire bio load... you beding does as does your water. As i am sure you know.
the concern is that once you remove you don't want a giant cycle.
I would have to say at least a month if not 6 weeks min..
And remember when you take it out to do water test twice daily. if you start to see a cycle... it was not ready to be removed.
your doing it right and your concern is dead on .... just wanted to toss a few things out to think about while you go through the process..
i have seen this happen and the effects are devastating to a tank if not prepared.