I've run into those type conversion problems before. The data is usually still there, it's normally that Excel has decided to use a certain display format for the cell (such as a date). Simply set the formatting back to "General" and all is fine. Usually.
It's possible I've missed some setting somewhere in the tangled web of menus, but I'm fairly certain the data is lost in the case of both those conversions.
For example: Type 09-2012 into a cell and hit Enter. It will probably turn into Sep-12 (or some other variation depending on your default date settings). If you convert that to Text or General, it turns into 41153 -- the number of days since 1/1/1900.
Excel recognizes all dates from 1/1/1900 through 12/31/9999, so it happily converts anything from 01-1900 to 12-9999 into a date. You can create a function to convert them back, but it would have to be based on the assumption that the format was originally ##-#### (which you may or may not know for certain depending on where the spreadsheet is coming to you from).
