That should be it, but since I rarely use UIbuilder and at least for me it often messes up something I can't guarantee that it will work.
You could do the change manually by editing the file
Everquest II\UI\<Your Custom UI folder>\eq2map2\core_mapstyles.xml in a text editor.
In the file you will find this:
<ImageStyle Name="qey_guildhall_tier2" displayname="Guild Hall Tier 2" menugroup="Karan" zonerect="-50.89, -26.69, 51.09, 112.75">
<ImageFrame Source="images/maps/map_qey_guildhall_tier2.dds" SourceRect="0,0,436,506" />
</ImageStyle>
Change the zonerect part and reload the game (actually /loadui works for this as well). Running the EQ2MAP updater will overwrite your changes.
In the future, if you create further maps then you can add the code above to
_User_MapStyles.xml to test your maps. That file will not be overwritten by the EQ2MAP updater.