You can't just send the judge a letter asking for that. What you actually have to do is file a Motion to Modify and it is substantially more difficult than just sending a letter since it is, in essence, starting the custody lawsuit over again.
You have to file the correct forms with the clerk, have a citation issued and the citation and a copy served on the other side, you may have to have an attorney ad litem appointed for the children (depending on the case), do discovery, etc. Some types of cases have to be supported by affidavits.
While you are allowed to represent yourself, if you have never done this before you should seriously consider hiring a lawyer. While there are forms online for most things, may of the forms have to be modified and unless you are familiar with the Family Code you won't know what modifications have to be made.
The other alternative, and less attractive than having a lawyer just handle the entire case but preferable to trying to do it all yourself, is to hire a lawyer and/or a paralegal to handle the paperwork for you.
I'm sure you have additional questions so please ask them in this thread and don't issue a rating until all of your follow up questions have been answered.