Several possibilities here starting with is this a Real Style (all Real Tracks) or a midi style (all midi tracks). Most Real Tracks will not play at double speed but some will. Midi is no problem, you can change midi tracks to any tempo at any time.

What I do is change styles as well as change tempo. Slow styles play best at slow tempos while fast styles will sound better at faster tempos. Also, many jazz songs use a latin feel for one part and a swing feel for other parts. Hit F5 and look at Bar Settings, you'll see style as well as tempo changes. If you want slow speed from bars 1-16 then double speed from bars 17-32 then back to slow at bar 33 use a slow style and set the tempo for the song itself then hit F5 and pick a fast style and set the tempo at bar 17 then back to a slow style/tempo at bar 33 and so on and forget trying to get one style to do both.

Now what else can you do here? You can use completely different styles for each song section. If you're talking swing there are dozens of slow/medium/fast styles to use.

To take this one step further you should also check out Real Band. In RB you can change styles for each instrument part or even part of one track and RB has 48 tracks to play with not just 7.

Bob

Biab/RB latest build, Win 11 Pro, Ryzen 5 5600 G, 512 Gig SSD, 16 Gigs Ram, Steinberg UR22 MkII, Roland Sonic Cell, Kurzweil PC3, Hammond SK1, Korg PA3XPro, Garritan JABB, Hypercanvas, Sampletank 3, more.