Mobile-CBL-Android-Bucket Merging

Dear Experts,

Currently I am using CBL in my Android (Kotlin) application where we have implemented only Pull replication. Currently we have 4 buckets in Couchbase Server as well as Mobile Couchbase-Lite database and syncing is working properly using SyncGateway. The app is stable and working fine from last 1 year.

Now we are merging the buckets on Server side like below:

Old - bucket1, bucket2, bucket3, bucket4
New - bucket1, bucket2 (so bucket3 & bucket4 will be merge in bucket2)

I did the respective android code changes for Couchabse-Lite database and things are working fine as expected.

My query is :
How to handle the existing devices in which four buckets are available?
After getting app upgrade, how can we remove the bucket3 & bucket4. For fresh installation there is no issue, but for app upgrade I need advice. Is there any way to remove the deprecated buckets? Please note - we have implemented PULL replication only.

It looks more like a app design question. So I assume you have to test your scenarios out. You are welcome to ask and API related questions.