AvinashR I really like the location tagging feature and seeing my posts on a map as I have lived in a lot of different places in the UK over my life. Admittedly I don't use it at much as I could. As a workaround to your issue, personally I would just go into the map to find the previous map pin and add the new one.
Coding-wise, I don't know how feasible this would be, but yes, a way of doing it could be a long-press on the currently assigned location, then in a new entry when you tap 'add location' Diarium could detect that there are co-ordinates in the clipboard and ask to add them to the entry.
Alternatively, a nice option would be a locations palette for regularly visited locations where you can add location presets (home, office, school, parents' house, in-laws' house etc) and assign locations from those presets.