"but now that BBI has announced that it is going to buy back...

"but now that BBI has announced that it is going to buy back SPARCS on market"

That is incorrect. It is the BBI Networks bonds (BBN020) they are buying back, not the SPARCS bonds (BBN010, although they did release this news under the wrong code on the NZX ie BBN010, when they announced this and this may be confusing you).

The BBN020's are part of the sweep facility (see the string on this from Wednesday) and this is why that particular buyback is occurring.

"so even if tommorrow's announcement states the results, it still needs to be agreed upon in May?"

Yes, and if 75% agree, there will be no conversions.

"Can they change their minds between tommorrows announcement and the meeting in May?"

No they cannot. (well you could try if you really wanted to). If you were smart enough to work this out, you would vote yes for the conversion and also vote yes to have the Trust Deed changed. If they get 75%, well you get another year to get a full return and 10% on the way, but if they fail to get to 75%, well you then get half of your holding turned into BBI shares and get to realise 50% of your investment for cash (if the market price holds up and assuming the number of conversions are less than 10%).

The ords share price will tank if they do not get to 75%.