After expiry date, can I downgrade license froum current version? Can I upgrade back to the version current in last month of upgradeable period?
yes, you can install any version that was available during the one year of license. so if you want, you can downgrade. but i strongly do not advise to do that, you will get more trouble, and will not receive support.
see here about licenses:
http://wiki.mikrotik.com/wiki/All_about_licenses