Steps are easy when the user need to be associated with the new LDAP server and/or Tree.

You create a new LDAP directory object in GW ( under system, ldap servers ) and point this to the new LDAP server
In system, directory associations you select the new LDAP server and make sure you set the option "Override existing association" next to the rest of the info.
In the Preview screen you can deselect users if needed and then run the associate
This should associate all the users found in the new eDir tree and GW at the same time.

So this should cover 1-3

Question #4 the documentation is clear on this:

NetIQ eDirectory and Microsoft Active Directory are fully supported for use with GroupWise. Other LDAP directories should work successfully if they use standard LDAP attributes. See www.novell.com/.../misc_popups.html

Not seeing much more for #5 however you can test the above with only a single user to see how/what it does before doing all users, just deselect all but one user in the preview screen

We are having 5000 users associated with old ldap (e-directory), so is there any way by which bulk association can be done.

For testing our new ldap we can add new ldap under ldap servers and associate one user with new ldap.

I answered that above already:

You create a new LDAP directory object in GW ( under system, ldap servers ) and point this to the new LDAP server

In system, directory associations you select the new LDAP server and make sure you set the option "Override existing association" next to the rest of the info.
In the Preview screen you can deselect users if needed and then run the associate
This should associate all the users found in the new eDir tree and GW at the same time.

And later:

however you can test the above with only a single user to see how/what it does before doing all users, just deselect all but one user in the preview screen

If this is not the answer im not sure what you are asking then

We created one test environment and done this testing at that time only 200 to 500 user bulk was possible but above 1500 bulk associate was not working.

I suggest you provide all details on this in the case you opened at the same time as this post as the main issue is the 1500 limit you seem to see

Include what GW version(s) you tried this in, are both LDAP servers eDir or is there also an AD LDAP involved, gwadmin logfiles created during the issue with more then 1500 etc so we can look into this more.

Our GW is 24.2, but we tested in 18.5. We will be going to test in 24.2. We are having DS 389 ldap server. Once we test with new 24.2 i will share the logs.