The question I would be asking is why?
Do they struggle to maintain your system so it would be advantageous for them to swap you to a 3300?
We have done the same for a couple of customers swapping 3300 to 5000 because we don't do 3300, we can achieve this if the customer takes our lines and calls so it's not actually free, you do pay for the system over the three years or so the lines and calls contract runs for.
I'm under the impression the 3300 costs more licensing wise, do you have a setup that may be expensive to expand, call centre maybe?