You do not need to purchase a license again. Your license is for life - it is only the upgrades that stop after 1 year with the Standard Edition.
If you have purchased the Standard Edition then you receive free upgrades for 1 year from your date of purchase. After that date, you can continue to use VueScan versions that were already released - but can't download new versions. To upgrade to the Professional version (which has free upgrades for life) click here.
If you have Professional Edition then you can continue to use and download new versions of VueScan for life.