David E. Kelley's tv shows are probably worth a mention here:
In The Practice, James Spader & William Shatner played Alan Shore and Denny Crane, roles they would later play in Boston Legal. However, The Practice's gritty (for a Kelley show) take on the legal world is some way removed from BL's very whimsical shenannigans. While I never saw any of their episodes of The Practice, I understand that their characters were very different from how they were in BL.
Additionally, John Larroquette had appeared in TP as a killer but later appeared in BL as a Carl Sack, a lawyer in Crane, Poole Schmidt, with no reference to the resemblance.
Chi McBride from Kelley's Boston Public appeared in Legal as his headmaster character Steven Harper. However, two other series regular actors from Public, Anthony Heald and Jeri Ryan, later appeared in BL as entirely different characters. Indeed, I think Kelley missed a trick here, as Heald's BP character had referred to his brother being a lawyer. Heald played a judge in BL - had they given him the same surname as his BP character, fans could have assumed that the brother had gone to the bench.
The Practice (Dylan McDermott era) had once crossed-over with Kelley's Ally McBeal, like Boston Legal, a much more fluffy take on the legal world, but even more so with its flights of fantasy and ghost characters. Two different continuities? Compounded by the fact that a regular Ally cast member, Peter McNicol, later turned up on Boston Legal as a different character. This was quite a shame as his McBeal character John Cage would have fitted in very well with Denny and Alan. Other recurring Ally actors like Armin Shimerman also appeared in Boston Legal but as different characters. Again, this was a shame, as both shows were set in Boston's legal world and both could easily have co-existed in the same universe.
Indeed, given that William Shatner's TJ Hooker inexplicably relocated for one season to Chicago, then back to LA with no subsequent mention of his sojourn there, this might also fit the bill. When you add in Star Trek: TAS, perhaps the Shat should get an award for playing three separate characters multiple times in different contintuities!