I certainly have noticed a change in the service which I am receiving from PlusNet. It is getting more erratic (sometimes takes quite a time to log on) and definitely slower. My speed test is the lowest (by 20%) than I have ever had before with this service, and sometimes it has almost ground to a halt.
By all means 'evolve' PlusNet - but try to get better, not worse. And I for one regard a premium rate support line as a disgraceful deterioration in this service you are offering.
For the record the premium support number is not replacing any of the existing stuff it is just an additional number for people to call about things such as how to setup cgi etc - things which the portal and google et al can answer if you're prepared to use them.