mode = active means "enable LACP only if a LACP device is detected".
Since cross stacked etherchannel doesn't support LACP, I'm guessing that neither switch is starting LACP negotiation, so LACP would not be activated and your network would work fine.
However, let's say that something happened to two of the three ports in the channel, so now you're down to one port, which means it's no longer a channel right? Would that then allow LACP packets to go out and prevent the etherchannel from coming back up later?
The other thing I'll say is that in my experience, even when everything seems to be working fine when you've set up a non-standard (explicitly NOT recommended) configuration, unexpected things can occur and you can spend unnecessary time troubleshooting those "unexpected things" because they don't follow any normal logic. Long story short, I would make the change at the next opportunity I had. Of course, if you have a quiet life maybe you're just waiting for some excitement :)
