unfortunately, 2 years later I am in the same place with a similar update.
I had 2 partitions. 50gb and 270gb
I ran out of space in my 50gb partition and had plenty of room on the 270gb
BOTH had data.
I partitioned the 270gb into 240gb and 30gb, with the intention of merging the 50gb and 30gb partitions together, creating a larger 80gb partition next to a 240gb one.
followed the directions provided here, and even though disk1s1, s2 and s3 were visible in diskutil list, once the merge command was issued, it combined ALL 3 volumes into a 320gb volume with no other partitions.
Ran the suggested recovery software but nothing seems to detect the original partitions after they were merged.
Just a head's up in case anyone else tries this.