Well, I'm going to assume that there was no written contract beyond the original proposal.
So, with no written contract the basis for your ongoing rate is simply that he's been paying that rate for years. The client obviously understands and is accepting of that rate as evidenced by the fact that he's used your work, asked for more work, and continued paying invoices all that time. So, it's certainly fair to expect him to continue paying your invoices at the known rate for work that he requests.
There appears to be no basis, however, for raising your rate because you 'felt they were no longer qualified to receive the reduced rate'. Unless this 'rush rate' was explained to them in some way, I don't think it's appropriate to suddenly raise your rate nor would I expect them to pay it, especially when the relationship is ending.
Do you think that the client noticed that you suddenly increased your rate? I know that I would.
Perhaps he felt insulted, too.
Perhaps you did put aside your other clients for his 'rush' job, but does that mean he would expect to pay more? I'm not sure, but unless he was informed about that increased rush pricing he wouldn't expect it.
Perhaps you could send him another invoice at your regular rate and hope to collect that $200+ so that at least you'll be paid a rate that is consistent with your historical rate.