Upgrade DB Server of old verion - any issues?

Hi,

I have a client who has some legacy machines (going to be decommisioned within 6 months) that still have SGN 5.5 installed.  These are now out of support and there is no intention to renew support or upgrade.

However, they need to keep these machines running for another few omnths.

I need to upgrade the SQL cluster that the back-end DB is running on to a supported version.  Currently this is on SQL 2005 SP2 and I need to move to SP4.

SQL 2005 SP4 isn't shown as ever supported for this version of SPN (but is for the next version 5.6).

http://www.sophos.com/en-us/support/knowledgebase/article/112780.aspx

A couple of questions:

• Are there any issues if I simply upgrade the DB Server?
• Can I simply move the DB to a stand alone DB server at the old SP (thus enabling me to upgrade the rest of the cluster) - any issues with moving the DB?
• What happens if I simply remove the back-end DB all togegther?  Do the clients continue to work or do they 'phone home ' occationally?

Thanks

Richard