I used to work for a SaaS company as well.

Whenever you need to make a change that people won't like, whether it be charging more (most common) or offering less, the answer is to simply blame the environment.

I'll write the communication for you for free. Also remind them that the purpose of them getting less is "so we can better serve you."

"Hello,
In order to better serve you, as of March 1st we will no longer be accepting paper checks. We are implementing this across the board in response to rampant deforestation and global warming so as to preserve the earth for future generations and because paper checks negatively impact minority groups and other marginalized communities more than others, some how. Plus paper checks could in theory spread Covid and we are supporting our irrational employees and their illogical beliefs."