Absolutley move the system. Business is long term and you want to be the guy there for the long haul.
Example:
I picked up a customer about 6 years ago and started with small repairs to their old Partner system. After my recommendations (a few years later after service calls, cabling, etc.) to move to a Magix and keep their current phone sets, their corporate purchasing department bought the Magix from another source......guess what....we installed it.....and have maintained it....

I could have been very pissed that they bought their equipment elsewhere

the end result......
they just called us to move their whole office
300 data and voice locs
move the phone systwem
overhead paging
security cameras with dvr

patience is a virtue !!!