Anytime you change the formula for a computed field on a form, you need to run a refresh agent of some sort on all the documents using that form in order for the new values to appear. In this case, it sounds like you can solve your problem as follows:
1) Create a agent called "Refresh Selected", setting it to run on Selected Documents from the Action Menu with the following formula:
2) Open an appropriate view and select all the documents affected by the form change, then run the agent by choosing it from the Actions Menu.
3) If you want to prevent indiscriminate use of this agent, you may want to change it to an "Agent List selection" type to get it out of the Action Menu. Then call it only from well-placed view action buttons with this code (Parentheses necessary when *calling* this type of agent, but don't put them in the agent name):
@Command([ToolsRunMacro]; "(Refresh Selected)")
One tip: If your form uses "regular" input validation formulas for some fields, your refresh agent will stop if it encounters any validation errors. I recommend avoiding that sort of validation for precisely this reason. I've written up an alternative approach on my blog here: http://www.lotusguru.com/lotusguru/LGBlog.nsf/d6plinks/KPET-6GXQBK
WARNING: Whatever you do, test this out on *copies* of 1-2 documents (or a copy of the whole db) before running it on all of the real ones...and take a backup then too. Depending on how your form is coded, you may recompute other field values that you don't want to, or in ways not intended.