The hardest part will be to get used to the tools, the different vendors have slightly different workflows. Overall, well written HDL stays the same. Ofcourse FPGA resources that are not logic, like RAM blocks, multipliers, MGTs etc need vendor specific templates to summon. If you want to keep your design portable, then its best to write a wrapper around them, synthesis will not insert additional logic. Basically if you write software in C for different platforms. Even there its sometimes good practice to try your code on different compilers and/or environments - keeps you sharp.

How well that works, you'll see for instance in projects on http://www.opencores.org/ not everything is gold there, but the Ethernet MAC is a nice example of a working IP core that works in more than one brand.

Cheers, Roger