Hi @hohmann,
I understand why you’re asking for this – but it would completely undermine the billing model. It could be abused to set a contact as marketing, send an email, set them as non-marketing immediately, and repeat the same steps for every single marketing send.
If that was allowed, everyone that’s aware of this hack would do it, leading to less revenue from HubSpot from marketing contacts – and that would inevitably lead to much lower marketing contact limits for everyone.
In other words, it would just be a matter of shifting the goal post.
In terms of your specific case, have you considered simply setting the forms to not mark submitting contacts as marketing contacts? (options tab of the form editor) You could review the contact, then decide whether they should be marketing contact and keep the ineligible ones as non-marketing contact.
Best regards