Your Residence visa continues in force all the while you stay in NZ - in theory, if you didn't leave, for life.
So you didn't need to update it and you're not in any trouble.
You're quite right that you'd have a problem getting back in if you left NZ, but this is easily sorted out. You're entitled to PR (in the same way as someone who got Residence through work and/or qualifications)
http://www.immigration.govt.nz/migra...quirements.htm and you just need to go through the procedure.
http://www.immigration.govt.nz/migra...teps/apply.htm It's a formality.