Thanks very much for your efforts. After reading your last email I
discovered the full stop in the file name was in the wrong place possibly
because "moneydancearchive" is an unusual file extension. Once I moved the
stop to the correct place and unzipped the backed up file I was able to
restore the old data.
Can I upgrade from version 2015 to 2020 without losing any data?
You will need to purchase a upgrade license at a 50% discount, but yes you should be able to upgrade straight to version 2020 without problem. I have tested data from many older versions, back to around 2010 and had no problems. That being said I would strongly recommend taking a backup of Moneydance before starting just in case.
Is there some way of getting rid of some of the old data without screwing things up?
The short answer is no. If you really understand how Moneydance works it is not impossible, but it can be a lot of work and I would not even think about it if Investments are involved. It usually however does not give the desired result. Many people wish to remove older data in the belief that it will reduce the size of the data set, Moneydance though uses a journal method of managing data so it has the opposite effect and actually increases the size of your data set.
The approach that is recommended in Moneydance is to do things like marking old accounts and categories that are no longer used as inactive, remove no longer used accounts from the Summary page and the side bar and to use the filters in the register to restrict the amount of data being displayed.