Thank you both for replying and for the kind welcome!
RTR, the 55 gal. tank does
not have an UG filter. It does have two Aquaclear HOB filters: a 70/300 and a 110/500. I use an extra block of filter foam over each intake as a sort of pre-filter.
I hadn't thought of this earlier but I bet I could just change the entire substrate out at once and then supplement the bio. filter with a dose of Biospira if need be.
Well it appears that it is crushed coral or aragonite for me. I asked about the Tahitian Moon sand because I love a black substrate in my tanks. I'll have to do a search for black aragonite...
Myaj, thanks for the tips. Good point about not cleaning anything for awhile beforehand. Also about keeping a nylon stockings of the old gravel in the tank for awhile after the changeover. I hadn't thought of that but it seems obvious now that you mention it.
Suddenly changing the substrate doesn't seem like such a big issue.
Another question or two, I hope you don't mind... Are the HOB filters going to be ok for BW/SW? Am I going to require any additional equipment as the tank becomes more brackish? Skimmers, UV sterilizers, refugiums, etc? I have
no knowledge of these things as I have absolutely NO brackish/saltwater experience, but I have been keeping fwfish for a few years now. I know I will need a hydrometer (or is it a refractometer? I'm dumb).
I raise Mystery Snails, Sterbai Cories, Red Cherry Shrimp, and Dwarf Bristlenose Plecos to supplement the hobby. I also keep a variety of others who do not reproduce. However, now that I have found the wonderful world of Puffers I am leaning towards phasing out some of the above in favour of keeping more puffers.

I currently have 11 tanks in use ranging from 20 to 55 gals each.
I truly want to provide a healthy environment for my sweet GSP "Mojo". Again I deeply appreciate being able to get advice from ye who are experienced and knowledgeable. Thanks again!