Lots of different factors here. Sometimes they change out turbines or impellers or the stator, sometimes they cut the tips of the stator fins in a lathe. sometimes they bend turbine or impeller fin tips to a different exit angle, sometimes they increase clearances between the components. It depends on the components used and the Converter company's "ideology" on converter design.
The reason I ask is because a had my converter tightened up a bit but it was tightened drastically. Now, when the lockup comes on I hardly notice it. Stall was at around 2K. Now it pulls my BBC 4x4 at idle easily. RPM's drop low when shifting almost to the point of bogging. I'm going to play with the modulator shift points to see if I can live with it.
I was hoping it was a simple as switching impellers so they could go up a step when I get around to pulling the trans again (not so fun with the 200lb+ cast iron transfer case).