I recently had exactly the same experience. I've been a TPG customer for 17 years and like you, I was on the same mobile plan which gave me $550 worth of calls and 1.5 GB of data for $19.99 with a $3 loyalty discount because i have my internet and home phone with TPG. I enquired about moving to the new unlimited plan but was told that if I did I would lose my loyalty discount. I spoke to several different people at TPG on several different occasions and each time i was basically told that there was nothing they could do about it.
I was so disappointed with the TPG that I decided to move my mobile to Vaya which gives me unlimited calls and SMSs and 1.5 GB of data for $14 a month. Within minutes of my phone number being ported from TPG to Vaya an $11 charge appeared on my credit card. I had a look on my online TPG statement and it is a charge for porting my mobile number. As I was only 14 days into a 31 day billing cycle, I called TPG to see when I would get a refund of the unused portion and was told that I wasn't entitled to receive a refund as I had already used my phone during this billing cycle, which makes absolutely no sense!
I was initially very disappointed with TPG for removing the loyalty discount. 17 years is a long time, particularly in an industry that hasn't been around for all that long. I am even more disappointed now, after charging me to port my number and refusing to give me a refund on the unused portion of my plan. If you do decide to move to a new service provider, make sure it is at the end of your current billing cycle!
