Problem with host application assembly versions

We have developed one add-in project base on VSTA integrated with our application[Host assembly version 1.0.0.412]. We are able to load/run add-in with our application. When we are planning to release new version of our application [Host assembly version 1.0.0.721]. Loading/Running existing add-in throws error that "Could not load file or assembly "host application assembly" Version=1.0.0.412"

We want our application to be capable of loading/running add-in built with older versions also.

Please let us know the solution for above issue.

Nitinumap,
Are you using VSTA v 1 for your host version 1.0.0.412 and VSTA v 2 for your host version 1.0.0.721?  If so you'll want to check out the Upgrade Helper.  VSTA v 1 and VSTA v 2 add-ins must be loaded differently- refer to the SDK walkthroughs / samples for the loading methods.  I would suggest using a different known locations to load the add-ins from.

If you are using the same version of VSTA for both host versions, then so long as you install both proxies in the GAC, the proxies are versioned, and ensure backwards compatiblitliy within the host you should be ok.  Here's a related blog on using multiple host applications/proxies and a sample which demonstrates two host applications (like two versions of one host app) sharing a host ID but using different proxies.

I hope this helps, please let me know if you have any questions.
Thanks,
-Melody