Just ask him/her politely for a raise. Just be natural to ask for a raise, after all he/she have said that you will earn more per word after quite some time.
Tell him you have done excellent job in writing him/her articles, and you feel that you needed a raise to sustain your expenses, etc. I'm sure he'll give you what you're asking for. Good luck.
When it comes to old and dear customers, asking option is better and that looks more convincing. It will not be a good idea if we take them for granted. Asking politely really works for old clients and telling works better for someone with whom we worked once or twice for a test, they liked the quality and now we can tell them about rates reconsideration.
Wow. Lots of feedback on this! I solved my problem guys. I ended up getting more work instead of more pay per piece. It's better this way because it isn't THAT much more time spent. I get a considerable amount of added income, and everyone wins. Thanks to all that answered. Hopefully this thread can live and help out other people who may be faced with a similar situation!
I enjoyed reading the different opinions on asking/telling/demanding.
In the immortal words of General Westmoreland in response to winning over a people's hearts and minds:
"Grab 'em by the balls, their hearts and minds will follow"
(I would have used a smilie, but some of them seem to be missing just now)