Yes, the upgrades available here in the A1 are individually purchasable for retrofitting your current SRS. Any SRS built in the last 6 months is already machined to accept the adjustable cheek. Older SRS's can have the handguard, and buttpad, but no adjustable cheek.
The A1 is an additional $350 over the base SRS. Individual handguard/buttpad pricing is not available yet.
A1's are scheduled to start shipping in march. Get yours reserved NOW if you want to get toward the front of the line. Once SHOT show kicks off... the line is going to go round the block.