If you aren't already charging market value it's highly unlikely you'll retain any existing customers once you do start charging market value.
Chances are, clients you currently have are clients because you aren't charging them what anyone else would charge. If you raise your pricing to more standard rates, these clients will most often just go find someone else that undervalues their work rather than paying you more.
Cheap clients tend to stay cheap clients... and don't really have much loyalty towards workers.
The best way for you to make more money is to find new clients at your new pricing.
The fact of the matter is that whether working full time, part time, or just spare time, your hourly rate shouldn't be any different. The difference is in the amount of hours you work.
All that being posted, some hourly discount due to turnaround speed may be appropriate. So I can see why someone would charge a lower rate to make up for the fact that they need two weeks working in their spare time, as opposed to 3-4 days if they were working full time. Still best to find new clients rather than trying to retain the lower paying clients. Raise your rates.. if they stick around great... but expect them to leave.