I think that PTSB were one of the institutions that said they would no longer be offering tracker mortgages so you may be pushed to a variable rate at the end of your discount period. I'd contact them now to clarify if you will be treated differently as you are an existing customer and that they will actually offer you a tracker (if that's what you want), although the tracker they offer may be prohibitively high.
If your current tracker is less than the NIB one ([broken link removed]), it might be worth keeping your current rate until April 2009 and then seeing what TSB has to offer and shopping around. However, if the rate is comparable to the cheapest you can get right now (I'm only using NIB as an example as they still have trackers out there and I use them so have their website on bookmark), you might be better off switching now and locking in the tracker margin with an institution that will guarantee the margin for the term of the loan. If you leave switching to April 2009, the tracker rates available even with NIB may be at a higher margin than currently.
Good luck with it
Sprite