Changing text field due to a messageBox entry

Basically, I have created two pop up boxes which

First Box: Asks the Client Name

Second Box:Asks the Renewal Month

Once these have been answered, the fields on the form, have the answers in them.   Cool.  Done..

Now I am hoping, that the answer in the Renewal Month text field, will make another text field change, depending on the answer.  (In the following examples, each Jan, Feb, Mar etc. are separate text fields.)

Example 1:

Client Name: John Smith

Renewal Month:  January

Jan     Feb     Mar     Apr     May     Jun     Jul     Aug     Sept     Oct     Nov     Dec

Example 2:

Client Name: John Smith

Renewal Month:  September

Sept     Oct     Nov     Dec     Jan     Feb     Mar     Apr     May     Jun     Jul     Aug

Side Question: In the message box, is it possible to make a drop down field, so they would have to choose one of the 12 months instead of typing their own, (and possibly misspelling the month)

Thank you in advance!