From this page, DTExec 2012 converts earlier version packages in-memory to 2012 format. It is conceivable that the conversion could fail, however, it should be possible to test the packages, and if they run successfully once (i.e. convert successfully) then they should do so every time. Their behavior should be similar to a high degree, but I would not expect complete equivalence in all circumstances.
The most likely scenario for conversion failure is if the package includes third-party components that are not available for SSIS 2012.
I would not assume that a package can be run by a later version of DTExec without testing, but if it runs successfully once, there is a high probability that it will run successfully every time.