If you were starting on a different airplane at a different airline, it would really depend on your experience level. No one is going to argue against that.

Changing airlines isn't really that big of a deal if you have a solid base of experience. The FAR doesn't change. The AIM doesn't change. Laws of aerodynamics are constant. Even reading a Jepp and flying a STAR/SID are the same. The OpSpecs for the most part are the same. The deicing procedures are very similar. If you have average intelligence and put forth effort to learn the new way of doing things, you should be fine if you are an experienced pilot (minimum of 5 years at the regionals).

Guys that have 7,000 hours TT, 5000 hours in a CRJ, and 7 years in the regionals should not have a difficult time upgrading at a different airline in under a year.