Go ahead and offer a pay alternative!
I think you deserve the income and
some people would be more than happy to pay a little.
But please leave the free alternative, with ads,
so anyone who wants to keep using your work the way it is can do that.
Offering a pay alternative, adds an option, it doesn't take anything away.
Go for it!
I am unlikely to pay, but would consider it,
if there was a simple once off charge, not a monthly fee.
What would I pay? Hmmm, maybe US$5 once off and forever,
would NOT ever pay US$1.49 per 3 months or whatever.