sliding speechange

I am looking for a sliding speedchange (not pitch), meaning the tone shall be the same as before.
I wout like to get the bpm/speed up from value x at time t0 to a value y at time t1 real smothly.

I know the sliding pitch feature in 1.3 but it effects the tone as well.

I tried as well the nyquist turntable warp plugin but it only works with effecting the tone as well.
So, is there any solution or hint out there?

Thanks!

The sliding pitch/speed change in Audacity 1.3.12 can change tempo independently of pitch. The quality is as about as good as you will get anywhere, but the current version is very slow and has a bug in it - make sure you back-up before you use it. (There is a new version being developed that is a lot faster but still the same high quality and subject to testing being successful should be available in the next release of Audacity).