Migrating from 3.6.20 to 4.0.20

Hi All !!
Hope somebody can give me a clue about the better way to solve a problem.
CentOS 7, standalone mongoDB server, initially 3.6.1.
I’m trying to migrate that into the latest and the procedure suggest to proceed gradually.
I did update without problems from 3.6.1 to 3.6.20 and now I’m trying to migrate from 3.6.20 to 4.0.20.
I did install the 4.0.20 binaries and pointed it to the same area where the data were stored.
When I try to start it I end up with :

ERROR: child process failed, exited with “error number 62”

Looking here (https://docs.mongodb.com/manual/reference/exit-codes/) says :

62 Returned by mongod if the datafiles in --dbpath are incompatible with the version of mongod currently running.

There are some procedure to follow to fix the problem ?
I’m searching but so far I didn’t find a clear answer or a procedure I can follow to fix this problem or also how to have more details about the nature of the incompatibility.

Thanks for any help !

Have you checked what was the previous Compatibility version on 3.6.1 & 3.6.20? before the upgrade.
This this setup running on 3.6 or migrated from lower version?

db.adminCommand( { getParameter: 1, featureCompatibilityVersion: 1 } )

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It seems I did find the problem.
I did re-run all the migrating procedure, the compatibility as before was set to 3.6 then I did restart the server.
I noticed that the old 3.6 was still running somehow, I did remove it but using an automatic procedure.
I did that manually this time, i.e. removing the 3.6.20, verified that only the 4.0.20 was present on the server and the 4.0.20 finally started without complaining !!

Thanks !

Hi !!!
The compatibility version is set for 3.6 as per instructions.
I don’t know what version of DB was used when everything was created.
I started from the 3.6.1, migrated to the 3.6.20 (still everything working) and then I started the migration for the 4.0.20.
The first time I checked the compatibility was set to 3.4 so, per instructions, I did set that to 3.6 while still running the 3.6.20.
Then removed the 3.6.20 and installed the 4.0.20 and I ended up with the 62.

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