Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
Do you still have the Goldfish?
If so, then they are keeping the nitrifying bacteria fed with the ammonia they produce.
If you change the tank decor, plant it, raise the temperature and turn it into a more tropical tank the bacteria will be fine, as long as you keep feeding them through the change. You could change it all over in just a short time, an hour to a couple of hours, depending on the size of the tank. But don't raise the temperature that fast with the Golds still in there. A degree or two per day is safe. More might be OK, or might not. Fancy Golds are fine with temperature into the mid 70sF. Comets and similar Golds are better not higher than the low 70sF.
If you have already removed the Golds, then keep feeding the bacteria with ammonia while you are waiting to make the change. If you add enough ammonia to read 3 ppm once a day the bacteria colony will be in fine shape when you are ready to add tropical fish.
There are a fair amount of nitrifying bacteria that live on other surfaces (besides the filter media) such as on the substrate, and other decor. If you are getting rid of these items then the filter will be the only source of bio filter for a while. Again, feed these bacteria just like doing a fishless cycle and the population will increase very quickly.