Not that
@jsker needs my validation

, but I agree that running both for a couple of weeks would be a good option.
There are several other factors to consider if you wanted to do it faster such as what is the size of your tank, do you have corals or is it a FOWLR setup, how much live rock/live sand do you have, what canister filter are you adding, what type of media do you have in the canister filter, what type of media do you have in the HOB, and do you use the HOB for biological and mechanical filtration?
Using a canister filter is a valid option for a saltwater tank, but does come with some opportunities. If you stay consistent with frequent water changes, clean your filter regularly, and keep up with your water testing you should be able to address most of those opportunities. Good luck with the transition!