All depends on what you currently have for equipment. But in case the equipment you currently have is set for a 90, and only a 90 then you will have to look into the following
Skimmer - will your current skimmer keep up with the extra load.
Sump - to gold extra water when you shut the return pump off.
Return pump - will the current return pump handle the return.
Flow in tank - How many powerheads and what flow rate are they. Are you going to have enough tank turn over with what you currently have.
Those are the main ones I can think off of the top of my head. I'm sure there are more, bit to me these are the most important.