Tracking then trimming

I've been cutting a show with a lot of monitor burn ins going into plates shot hand held. They all need tracking and depending on the shot I've tried a variety of methods from Mocha to Boris to -- most of the time -- Avid's own tracker. The project is still a work in progress, so it's not unusual for me to track material into some of the monitors only to decide later I want to trim the shot. I get it that if I lengthen a tracked monitor shot, I'm going to have to re-track. But it seems that I'm not able to shorten a shot without losing the tracking. Is it the case that once a motion track has been created it always starts from the first frame of the cut even if I change that first frame? In other words am I correct that shortening a tracked shot will always throw the track off or is that dependent on the kind of tracker? I would have thought that if I shorten a shot, the track data would shorten in sync, but I must be wrong. Can anyone clarify? (BTW I just used the new feature in Mocha 2.6 that allows you to export tracking data to the Boris Corner Pin effect. I had a pretty complex track -- a hand held and sped up pan across five of those cursed monitors with no more than two in frame at any one time. It was a long process to build all the Mocha tracks, but I could then import them into Avid via the Boris plug in and it worked wonderfully -- until, of course, I decided I wanted the pan to start a little later and sure enough all my burn ins were floating in the frame -- curses). Thanks for any insights.