Hi Yakov;
I used to do that too but found that scratching activity accelerated my hair loss. 
The Migrate option is valid. You can perform that activity in the PB Classic IDE via using the RHMB on the Target object in your System Tree. Then from the resulting pop-up menu, selecting the "Migrate" menu item.
Now the purpose of the Migrate after the fact covers the situation where your developing an application in lets say PB 12.6 and then realize that you would like to use a few objects from an application still maintained in PB 11.5 for example. The developer can copy the classes from the PB 11.5 PBL to a 12.,6 PBL via the PB 11.5 IDE. However, the copied object's source code is still in PB v11.5 format.
In order to make sure that all your PB 11.x objects are ready for use in the 12.6 application you now need to select the "Migrate" option from the PB 12.6 Target. Any object's source that is in 12.6 format is ignored while the 11.5 object source is migrated (upgraded) to the current version. After the migration, a full build step is also automatically performed by the PB IDE to make sure that the linkage-editor is OK with all the class references.
HTH
Regards ... Chris
PS: I have used this procedure extensively for the above scenario since the PB 3.0 days.