Demi Lovato & Nick Jonas We Can't Wait To Tour Together ...

10/27/2015 8:38 AM PDT

Demi Lovato had no guilt over 86ing Adam Lambert from her upcoming world tour, hopping onstage with Nick Jonas for a private concert Monday night in New York City.

The tour's still 8 months away ... but the two were already warming up for what's to come. 

TMZ broke the story ... Demi ditched Lambert in favor of Jonas at the 11th hour.