ug filter is short for undergravel filter.. if you take the pipes out, (it shouldve come with caps when you bought the ug system..if not then you should be able find replacements at your pet shop), so cap it up, and youre done.. so in other words take out the pipes, leave the ug plate.
a plenum system is a low to no oxygen zone where anaerobic bacteria turns nitrates into nitrogen gas, which means the end result of the nitrogen cycle..
aerobic(this meaning high oxygen areas) bacteria can take care of the ammonia->nitrite->nitrate process, but cant take care of the nitrate->nitrogen gas process, it requires a different type of bacteria(anaerobic) to do this. so as you know anaerobic requires low to no oxygen areas, which is what your plenum will accomplish.