I have an app that was developed for my company. Unfortunately it needs to be resigned (i know how do this part) because the distribution cert has expired. The problem is we
In answer to Chris Emerson's comment above (sorry, I don't have enough reputation to add another comment so I'm adding an answer) it is still possible to do this! I have just done it for an IPA with latest updates to everything iOS 8.3, OX Mavericks. I was getting the same message "... could not be installed at this time" and it turned out to be an issue with the entitlements in the provisioning profile not matching the entitlements in the *.app.
You can check the entitlements in both like this: https://developer.apple.com/library/ios/technotes/tn2318/_index.html#//apple_ref/doc/uid/DTS40013777-CH1-TNTAG68, and I put an answer detailing how I fixed the entitlements of my IPA so that they matched here: Alter Minimum OS Version of IPA without XCode, iOS 8+.