Here are some tech notes regarding the issue:

DOCUMENTATION: What is the process for decommissioning a NetBackup 6.5 media server? 

NetBackup Media Server Decommissioning Tool now for 6.53 6.54 and 6.5.5

Changing from ACSLS to TLD should not cause any issues unless you are using ACSLS access control or volume control.

Reconfiguring the devices is not the major concern here - it's taking care of media and images owned by the decommisioned media server.
On the main page of this forum, you will find this link under 'Featured discussions' :
NetBackup Media Server Decommissioning Tool now for 6.53 6.54 and 6.5.5

As far as conguration is concerned, you need to delete Storage Unit and all devices belonging to the old media server, use the wizard to scan & config devices and Storage Unit on new media server and update Storage Unit selection in policies.

thx mari and nicolai
what if i keep same hostname do i need to use decommissioning tool

another question
what about replacing hba , what steps and what risk should i take on consideration

I would go for adding the new Windows media sever and then re-assigning the media information to the new media server. This approach let you the new media server gracefully take over new load. And you have a fall back option if you experiences issues.

Replacing HBA: Do you have a fibre channel HBA in mind ?

Fibre channel network can be zoned by switch port or by World Wide Name (WWN). If the  zoning in your'e fabric is done by WWN you need to re-configure that zone. If zoning is done by switch port, no re-configuration should be necessary.

thx nicolai
i will do the re-zoning
but changing HBA card , will it change hardware path for tape devices
so do i need to reconfigure STU
what ive ACSLS host

If you keep the same hostname, you won't need the decommissioning tool. Delete all devices for the AIX media server before switching it off, and rescan for devices after installing NBU on same hostname Windows media server. You will have to delete the STU and re-add with new robot details. If the STU name is different, remember to update policies. Be sure to test restores after the move, as I'm not 100% convinced that a Windows server will read AIX tapes without any problems.

As Nicolai said - a phased migration might be better. Add the Windows media server with a new hostname. Move tapes and media ownership (using bpmedia) gradually, testing restores after each move.

Marianne raises a very important issue. If you have tuned SIZE_DATA_BUFFERS (everybody REALLY should) to a value larger than 65536 you absolute need to make sure the Windows miniport driver support SCSI block sizes larger thank 65K.

Tech note:

Where are the values for SIZE_DATA_BUFFERS, NUMBER_DATA_BUFFERS and NET_BUFFER_SZ located on Windows NetBackup Servers?