There is no built in tool for such a batch field change.
There really is no need for such date specific field names as you have here. The date data is entered should be entered into a field of type date, not made part of a field name.
Sorry I was unclear. The DATA in the fields isn't dates--it's amounts, yes/no, sizes, etc. The field names separate this data for each month of orders (we run a publication, so these represent ads in each issue). So there's a "Jan11 size" and a "Feb11 size" and a "Mar11 size" and so forth...
You were quite clear. Apologies for not being clear in my resonse, The Date should also be entered in a date field along with the other data that you are entering.
I believe you need a set of related records instead of a set of customized fields (and matching date specific field names) here.
Each add would be a separate record with a date field (January 11) and then the associated fields for amounts, sizes, etc. For a new add, make a new record, enter the appropriate date and then fill out the other fields. No need for batch changes of field names that way.
Since this is for ad placement, you may not even need the date field. You may want to create a table of "issues" where each record records the publication date of that issue. Your ad records would then link to the issue or issues and list the appropriate date information from the related Issue record(s).