Answer

This behavior is intended to avoid breaking your permalinks (as in, if someone linked to your post and then you re-named it later). Movable Type uses the Entry Basename field by default to name Individual Entry Archives. The basename is assigned (from the entry title) the first time an entry is saved, and this field is not updated merely by changing the entry title.

Assuming you're not concerned about breaking inbound links, there are two ways you can override this behavior.

(Please note that when you change a title or any element that modifies the file name like this, you will need to remove the old version of the file from your server manually.)

Change The Basename

In version 3.2, you can change the basename which was assigned to an entry.

Navigate to the Edit Entry screen for the entry, and scroll down to the Basename field. Click the lock icon to the right of the field to unlock it. Enter the new basename you desire for the entry, and click the SAVE button to rebuild the entry.

Warning: Currently, the export format does not include the basename field. This means if you use this method and you then export your entries, when you import them into a new weblog your assigned basenames will not be carried over.

Use An Archive File Template

If you wish, you can override the default file naming behavior by setting up an Archive File Template for your Individual Entry Archives (do this in Settings > Publishing : Archive Mapping¹):

<MTEntryDate format="%Y/%m">/<MTEntryTitle dirify="1"
  trim_to="15">.html

Save the change and rebuild your site, and this should create a file with the new entry title.

¹ In versions prior to 3.2, navigate to Weblog Configuration > Archive Files.