Best guess, going to a Skeet (5 POC) from a Mod (20 POC) will give roughly 4" more usable spread at 21 yards, the longest distance of a shot at Skeet.
That's not inconsiderable. I'd also think about going to Cylinder instead, you may pick up another inch or so over the Skeet pattern.
Many choke makers consider Skeet to be 5 POC, but not all. Remington has a Skeet tube LARGER than bore diameter by 2 POC, more open than Cylinder.
Choke tubes are not that expensive, so a little research won't break the bank. Try a few things out and see what's working for you.....