Are you using the same filter in there?
Are you keeping them out of the 29G tank in case of ammonia spikes from the soil? Otherwise you can put them in whenever you are ready. You will have BB from your old filter pad.
Also if you don't mind waiting a few weeks to rescape, you can add the new filter to the existing tank and get the BB to build up on the pads, I use two pads in my HOB
for double the BB action. Build the BB first so you can use the filter pads in a bucket for the pleco with a hob
and also on the other tank with the other hob
Plecos are pretty poopy so you would need to be very very careful.