And u don't have to take out your fish. I'm changin subtrate fish in.
The key question there is how long has the tank been established? If the UGF has all the build-up and nitrates trapped that everyone is claiming, then just ripping out the UGF and releasing that into the water could indeed cause some issues.
@ the OP,
I will agree that UGFs filters don't work well in a heavily stocked, heavily planted or over stocked system. However, they do work and they work well if they are set-up properly and properly maintained. I've been running them for over 20 years in multiple tanks ranging from 10 gallons to 100 gallons mainly stocked with African cichlids and I have zero issues with toxin build-up or out of wack water parameters.
Proper set-up requires the use of powerheads rather than air, and multiple powerheads on the larger systems.
Proper maintanence requires that you gravel vac on a regular bases (I recommend weekly with your regular PWC).
In my personal experience, I've found UGF to be less expensive, less messy, definitely less disgusting than a canister filter, and overall easier to maintain than any of the many different brands and types of HOB and canister filters I have tried over the years (and yes I have tried lots of different brands). And I have found them to be just as efficient at maintaining water quality as any of the new filtering systems.
Do they have their disadvantages - absolutely. Are they without flaw - absolutely not. Do they work - yes. Will they work in this particular case - probably not.