I think one problem is things like sum(tau_i, 1). That should be sum(tau_i; dims = 1). The error isn’t the most helpful here; what’s happening is that with the newer version of sum, the first argument is treated as a function to apply to each element before summing, but since tau_i is not a function but an array, it gives you that message. (While these kinds of errors can be frustrating, my understanding is a lot of effort was put into Julia 0.7 to give good deprecation messages to prevent the frustration!)
Edit: you may also run into differences if you are using a different version of JuMP now than you were with Julia 0.64. So it might be a good idea to look at the JuMP repo as well for changes to JuMP over the past few versions, or to make sure you are using the same version of JuMP and do the Julia update first, and then change versions of JuMP and update the JuMP code.