It depends on which Tamron lenses and mounts you have.

If they are older Tamron lenses with a true interchangeable mount system, such as Adaptall or T-mount, then swapping the mount unit can be possible.

If they are standard dedicated mounts, especially electronic mounts, the answer is usually no. Canon and Pentax versions may differ internally in mechanics, aperture operation, and electronics, so changing only the rear mount often won’t produce a working lens. For example, FD and Pentax K mounts handle aperture linkage differently.

A quick clue: if the lenses are built around a universal mount system, the mount is meant to be removable. If the visible screws and mount parts differ between the two lenses, that suggests the internal construction may also differ, making a swap unlikely.

So: possible for older universal-mount Tamrons; generally not practical or reliable for dedicated Canon/Pentax versions. The exact lens models are needed for a definite answer.