It is about time I started posting some workarounds I have discovered so others can benefit. Apologies to all that may have come across this too late.
I received the following error when deploying to a SSAS 2016 tabular model which was originally a 2014 Tabular model. The solution and SSAS project contained several solution configurations.
Error when deploying metadata:
"The Upgrade element at line 7, column 90 (namespace http://schemas.microsoft.com/analysisservices/2014/engine) cannot appear under Envelope/Body/Execute/Command."
I won't go into my general frustration with Visual Studio SSAS Tabular projects seemingly ignoring solution configurations such that when I want to deploy to staging, it should deploy to one server and when I pick the Solution's Production configuration it should deploy to another server. BUT, this did nuisance did ultimately lead me to my problem.
I had several solution configurations, one for staging 2014, production 2014, staging 2016 and production 2016. Each configuration had a different deployment server, but I had to open the Project in a Text viewer to confirm this since the Visual Studio UI seems to only display the 'last' deployment server selected. What I ultimately chose to do was while in the text editor with the SSAS Tabular project file, reset all deployment server values to my desired SSAS 2016 host, save the file, open the project in VS again and select Deploy. At this point I had success.
My only guess here is that under the covers, VS must have been using one of my original SSAS 2014 configurations I created and I was trying to deploy a 2016 Tabular model to a 2014 SSAS Tabular server.