disable upgrades in membership plugin

Hi

I have two version of the same site. In one version, when a member is due for renewal they have the ability to 'upgrade', in the other version when they are due for renewal they do not have the ability to 'upgrade'.

The thing is, from what i can see, both sites are set up the same. I would like to disable the ability to upgrade although i cannot see how to achieve this. Please help, i have been around in circles.

Thanks