Hiding a field

I hope this isn't asking too much. I would like to ask another question.
I am now able to calculate the Membership expiration date (from my last question. Thank you!). Her are the basics of the form I am using.
I have a Payment type (cash, check...), a Payment Purpose, and an unbound field for number of years that a member is renewing for, so far all calculations work and the data is updated correctly.
Problem... When the form opens, I only show the payment information (the number of years the member is renewing for is hidden, this is a general payment form). When the payment purpose is for "Dues" I now show the number of year box. When I enter the number of years (this is the last entry), I want the form to reload and re-hide these fields. I get an error that I cannot hide a control that has focus. I have tried most every field as well as form event, but I cannot get the field to re-hide after an entry is made and accepted.
Any assistance would again be greatly appreciated. Thank you for your patience and help.