Recently I had a customer who wanted to change the URL of his MySite Application in MOSS.
He had the MySites running with a non-standard port, for example 1234:

He wanted to change the URL to, let’s say testapp.local.
Here is what you need to do:

1. Extend the MySite Web Application

2. With the extension of a Web Application, you create an additional Alternate Access Mapping entry. You have to edit the entries for the MySite Application:

3. It should look like this when you swapped the entries:

4. Make an IISRESET to apply the changes.

6. Now your MySite Link should point to the new URL with Hostheader:

That’s it!