No need to upgrade your 12.8 agents on Srv2003 to 14.0sp2. 12.8 agent connected to 14.0sp2 domain will work fine including Remote Control. Srv2003 does support 12.9, so you could upgrade those agents to 12.9 if wanted to be on last supported version.
What may get tricky is using agent auto upgrade feature in 14.0sp2. If you use that feature one approach create a upgrade settings configuration policy linked to dynamic groups of which OS are permitted to auto upgrade. Your XP/Srv2003 OS wouldn't want to target with an auto upgrade.
Ran into similar situation when we upgraded from 12.5SP1 to 12.9 with XP agents. XP only supports up to 12.8, but we skipped 12.8 for 12.9 and let our XP agents on 12.5SP1 agents with 12.9 Domain, now at 14.0SP2 XP/2003 legacy agents still ok.