Drop/Create MongoDB TTL index vs. collMod in production
The first you need to do is calculate how many documents the index will delete if you change the 60 days into 30. That is a simple count with {TTL_FIELD: $gt:30days}. 

If the number of documents is big (how "big" depends from the server specs and the workload) I recommend to run an incremental Colmod like (pseudocode): 

While achieve 30 days {

NUM = 60 days;

NUM = NUM - threshold;

db.runCommand( {"collMod" : "COLLLECTION" , "index" : { "keyPattern" : {TTL_FIELD : 1} , "expireAfterSeconds" : NUM } } )

wait 2 minutes;}

If the number of deleted document is small just do the collMod at once or on 2-3 steps manually. 

Your approach to drop and recreate the index takes more time and if you have lot of documents for deletion you will get performance degradation when the index completes.

Hope that helps