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.

Standard Edition customers:

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.

Professional Edition customers:

If you have Professional Edition then you can continue to use and download new versions of VueScan for life.