Your SIA licence will not renew automatically. You must apply for a renewal if you want to keep working in the same role when your current licence expires.
You are not guaranteed a licence just because you already have one. This is because:
You cannot renew a licence unless you have a current licence in that sector. If your licence has already expired, you must apply for a new licence.
You can apply to renew your licence four months before your current licence expires. You should submit your renewal application as soon as you can. Applying early allows more time to process your application, which reduces the risk of you being unlicensed and unable to work.
You will not lose time on your new licence by renewing early. Any time left on your current licence will be added onto your new one. Your new licence will be valid from the day it is granted.