OTA could do the job, but that's a lot more work. The UI for ALM has the ability to do this for you with just a few steps.
All you need to do is set a filter and filter via your udf field and its current value, then select all results and do Update Selected to change your value.
1. Set the test plan view to Grid view.
2. Click on the Filter icon and choose Set Filter
3. Find the udf field and use it to set your sorted results (for value you wish to change from)
4. Once filter is set and you have your results, do cntrl A to highlight all results (or can via the Edit menu option use ''Select All'')
5. Right click on the highlighted results and choose the Update Selected option.
6. In the Update Selected pop up window, select Application for your Update Field and then select the Value to be changed to
7. Click Update and it should update all the tests from the filtered search