This is definitely a strange tactic. ACF has been under-priced (considering the lifetime, unlimited site term) for years, and I can understand new ownership wanting to create recurring revenue.

If they were going to do something like this, it may have been better when the acquisition first occurred. Write a blog post explaining that, while they will abide by the lifetime licenses, there are a lot of costs associated with continued development.

Show customers exactly what paying the annual subscription fee does in terms of funding the product and support staff. Then offer something of value in return. Maybe it’s a discount or free year for another product.

Be as open as possible. Otherwise, this doesn’t look flattering.