I agree with this guy ^^^
Let them know this is happening and why it's happening. Let them know a few times. When they reach out for a job tell them, "hey we increased our rates because of X Y and Z but I can give you this job at our old rate (or at a discount) because we are so grateful for the support you gave us to get this far.
You should be showing up in their emails weekly or monthly anyway so they'll be used to hearing from you. Also come from it from the persepective of this increased rate is what it takes to get it done, not I need more money for myself. I had a mentor once tell me - "the main function of a business, is to STAY IN BUSINESS". You can't do that if you aren't charging enough.
