If one has used PIC uC, how different is it to migrate to using a different uC like say Arduino or ARM?
All microcontrollers can be programmed in a C-like language, all of them have the same basic peripheral types and comparable cores as far as features go. When you stay within the realm of real-time MCUs, they are pretty easy to jump between.

Probably the biggest differences are simply in the idiosyncracies of the tools. If you are accustomed to the ease of having preprogrammed bootloaders in your AVRs (down to pre-programmed USB bootloaders in the -U parts), you may have some trouble with having to buy a compatible JTAG programmer for certain ARM chips. If you're accustomed to the rich debugging features of JTAG-enabled chips, it may be a bit of a culture shock to go back to in-bound debugging in low-end PICs and AVRs. But that's something you get used to after using a toolchain for a while. Also, with some chips there is quite a significant cost associated with using their tools, e.g. Freescale and Renesas.

Don't be afraid to choose a different chip once in a while and learn to work with it. It will broaden your view and enable you to use more optimal MCUs in the future

Addendum: this doesn't really apply when you go to non-realtime processors like the ARM A-series, x86 and Cell. These are very difficult to program for on a low level, tools are expensive and the chips are hard to get and implement.