I find this topic to be a little unclear. I have seen seemingly contradictory information on this topic.
At the link below, it clearly states "It is still possible to execute queries defined with BEx Query Designer 3.5 by using the analytic engine from SAP NetWeaver BW 7.4."
Alternatively, I have seen a number of assertions that suggest this is not the case, including a statement made by yourself on April 3rd stating "Dataflows, etc. will still work, but not your old 3.5 Queries in 7.40, they will work only until 7.31."
Upgrade to NetWeaver BW 7.30 - Technical Aspects
I am aware that a certain amount of remediation would be required as a result of a BW 7.4 upgrade, but it is still not clear to me what the full impacts would be to BEx 3.5 queries.