Yes I was thinking that.. but you asked about a painless script so that is the answer you got 
That painless script will update all the documents...
Perhaps look at this thread... the last couple posts (It is a bit long... but the OP is loading Geo Data from Postgres using Logstash)
Last Couple Posts..
I do not know pgsync... but in the end you just need to put the lat / lon in the right fields... I would think you would be able to do that... I just not familiar with the tool
Looks to me like you will need to use the transform node rename logic to rename your source fields to the correct fields..