Re-routing any ticket will require it to be reissued. Reissuing may attract an administration fee depending upon the territory. Re-routing onto another carrier will never be a straightforward exercise. The ex-TIP fares tend to be IATA fares which permit multi-carriers but only, I understand, if the relevant carrier does not issue a published fare for the route. If you want to change carriers, I suspect the easiest thing to do would be to cancel and rebook.

Re-validating - which is simply changing the date/time of a flight but keeping the same routing - used to be done by sticking a yellow sticker on the paper ticket with the new flight details. BA ceased using these stickers (at least in the UK) over 12 months ago. Some stations put the ATB through the machine and the revalidated flight details are then printed underneath the original details. More commonly, they just don't bother. If the reservation has been updated, they just take the original ticket without any amendment.