EOSL for NBU 8.1.x will only be determined when the next NBU major/minor version is released.
Select 'NetBackup Enterprise Server' under * Product over here: https://sort.veritas.com/eosl
As far as migration of NBU master from Windows to Solaris/Linux or any Unix version is concerned -
this is a Consulting engagement that can only be performed by Veritas consultants or a handful of partners that have received special training.
See:
DOCUMENTATION: How to migrate the NetBackup catalog information from one platform to another (UNIX to Windows or vice versa), rename a Master Server, cluster an existing Master Server, or merge multiple NetBackup domains.
http://www.veritas.com/docs/000026956
Replacing media servers is not a 'big deal'. Simply deploy new media server and then decommission old media server.
As far as OS Compatibility is concerned - there is no indication of Veritas dropping support for Solaris any time soon. This could change if Oracle decides to change direction.
See NBU 8.x OS SCL: http://www.veritas.com/docs/000115691
When support will be dropped in a 'next' version, we normally see something like this as a footnote:
NetBackup #.# and later does not support this CPU architecture
I would personally go for RHEL, rather than Solaris.