If the filter has enough material on it to cause blockage, that means you did not maintain your transmission properly and a drain/fill will simply increase the chances of breaking those little shavings free and getting them stuck in one of the transmissions many intricate passageways. This is why I recommend not to touch your transmission at all if you don't plan on changing the pan filter, because you're just going to have more problems.
^^this represents 1/2 of why you should not touch the tranny (if never serviced) past 100k miles. The other half is that the metal shavings (ARE) beneficial to the system.
Over time, Larger bits are stuck on magnet/filter. Smaller bits become crucial lubricant for moving friction parts (now with gaps due to metal shaving). When removing the fluid/filter you may:
-Agitate the Larger bits to cause clogging.
-Moving friction parts has these gaps which rely on small metal shavings. Not having small shavings means mechanical failure risk.