Your current mortgage will be cleared on the sale of the property against which the mortgage has been raised...
When you purchase a new property and assuming you will need some or even the same level of mortgage to complete the deal - then that will be deemed to be a completely new deal...
My suspicion is that you will make the argument (which is a very fair one from your point of view) - that you only want the same amount again on the same terms - the lender however will look at it differently and you will not be offered a tracker option...
pjmn