@Virgo 

Thank You so much! As for OC200 replacement with OC300 it is absolutely clear for me now.

Could you please answer one more question regarding the upgrade of the TL-R605 V1 router with a new one. Should I upgrade (replace) the router first, prior to the replacement of the controller or after I replace the controller? What in your opinion is preferable? As I understand the settings of a router are saved in the controller's backup file?

Thank You!

  @Virgo 

Thank You so much! I did everything tonight. First I have connected only the new router and controller and my windows notebook via a spare unmanaged switch, set up a permanent IP address for controller, restored the new controller with the backup file from my old one, amended a number of entries, passwords in the new controller, rebooted everything and disconnected. After that as per your advice I "Forgot" all the devices managed by OC200 controller (starting from access points first, after that managed switches and finally the old router). After that I disconnected both my previous router and controller, connected the new ones, powered them on, logged in to the new controller, adopted all the devices and everything is OK!

Thank You again for your help.