It also depends on the plants you want to grow. Things like Amazon swords and other heavy root plants will want a good substrate to grow in so the UGF won't be a good idea. Something more like java fern will work fine as it mostly uses it's roots to hold onto a surface. But all in all a UGF usually isn't a great idea with a planted tank and it will be best served comign out, the sooner you do it the easier it will be to get things settled. But I wouldn't do the switch until you know what sort of susbtrate you are going to use to replace it with and have that all ready to go. Do the change over once.
Any more questions on that will be best for the planted forums.