This is probably too late for you, Nadia, but I was able to solve the problem (which I encountered) and thought I would include in case others had the same problem.
After replicating the datasource in BW as a 7.x datasource, I went to transaction RSDS and entered the datasource.
I used menu path Datasource > Restore a 3.x Datasource where I selected "YES" for the pop up.
I have the same issue. I tried to work around it by using the parameter specified in this OSS Message: 1885443 - Replication of Datasources.
You must enter the RSADMIN Parameter by running the program SAP_RSADMIN_MAINTAIN with transaction SE38, in the fields:
'OBJECT' = 'REPL_DATASOURCE_UI_740'
to use the new replication the field:
VALUE = 'X'
to use the standard replication:
VALUE = ' '
Sadly, BW went ahead and replicated all datasources as 7.x even using the old UI. I was expecting to be prompted similarly to how we would in 7.0 to 7.3 versions, but the system just went ahead and activated all datasources in 7.0 version.