Change Api2 to Api3 is almost the same as create new ft.
From all exist geo add-ons for EE 1, you can try Geotagger (possible you need to speak with Mightybigrobot). I don't have last version, but I believe that they was able to which to API3.
If they have this version, you just will needed to split data from current SL Google Maps to Geotagger fields (they have separate field for lat & long). Something like
UPDATE exp_weblog_data SET you_new_field_lat = SUBSTRING_INDEX(you_old_field,',', 1), you_new_field_lng = SUBSTRING_INDEX(you_old_field,',', 2)
Plus you will need to make small change in frontend using any of exist gmaps jquery plugins.