Netbackup Compatabilitiy Version Compatability with Data Domain OS
2.6.2.0-410681 is still supports with your new patch 5.4.5
you can also upgrad the OST plug in to 3.0.x if you would like to .. that is also supported.. since you are going with DDOS upgrade. i would suggest you to go with OST plug in as well..
Yes, you can do that but just for your information, NBU doesn't really care about the DD OS version, the only thing that is important is the plug-in version and whether it is compatible with NBU - see https://symwisedownload.symantec.com/resources/sites/SYMWISE/content/live/SOLUTIONS/76000/TECH76495/en_US/nbu_76_hcl.html?__gda__=1431105221_9cd049a45e5acfebdb225b1a496a2b76#ost_secondary_storage_solutions-ost_storage_servers-ost_storage_servers_-_vendor_compatibility-data_domain
EMC provides a compatibility mapping between DD OS versions and Boost plug-in versions, you can obtain that matrix from the same place you download the Boost plug-in from.
