For the whole project:
select all tracks in the project, right click one, find
Set timebase of
selected track and choose [i]Beats (position,length,rate)[i]. Now simply
type in the new project BPM and everything will get stretched accordingly.
Per track:
Same as above, just set all other track's time base to "time" (so they
do not get stretched).
Per item:
Right click the item > Item settings > assign the proper timebase.
Vote for
bpm conversion dialog here if you feel you would use it.
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