I think part of this challenge is back to why customers on the 3.5% plus rates have not switched providers at this stage. If it is due to LTV in particular, and those above the Central Bank guidelines of 90% or 3.5 times income, then they could be considered a higher risk than new customers who meet the Central Bank guidelines.
If its down to pure apathy, or lack of trust or whatever, then this is a matter and I am not sure what the central bank or anyone else can do on this right now. You cannot force people to switch mortgages if they don't want to.
Most, if not all, of the those who qualify as a new customer (Central Bank guidelines) can switch provider if they wish to do so. They have the power to change banks behaviour by switching, and if sufficient numbers do, the policy will change pretty quickly.