how to deactivate order type?

Hello SAP colleague,

What is the best way for deactivating work order type which was in use for long time. Need to say that we still having a lot of work orders with this particular type 'in process' and a lot of them are already business completed.

What I tried to do is break the assignment between order type and plant but this is not the best solution because after deactivation

I could not open/display any existing work order with this order type. Removing it from number range is also not enough.

Thanks for all clues in advance.

Regards,

Grzegorz