Your Trans is on Borrowed time... I'll just about Promise that one to you... Changing the fluid may have sped up the failure as it may have removed some of the varnish or material that was holding that last little leg.. You would have found the slip sooner or later..
I can't say for certain how long you'll get out of that trans... But I'm thinking a couple of more months possibly...
Get the adjustable modulator, and give it a turn( Clockwise ) install it and see what happens... If its still slipping, then chances are the trans is not happy at all..
It should be fine with a couple of turns and function with a firmer shift.. If at the point you are still slipping, its time to look for a new trans or a rebuild..
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