i doubt they can force u off the $50 plan as it is separate from the data plan (some voice plans have data built in)
the value pack, specific for a blackberry, they will have to get rid of (since data is built in) and offer you standalone data
right now the 6GB for $30 promo plan seems to be back so you can go for that, or any other standard offer
they used to have a $10 value pack which included call display/voicemail/2500 txts but i dunno if they still have it available (this is a retentions value pack, regular CSR can not offer it)
bottom line is you should just ask to talk to customer relations (retentions) as your plan is customer relation specific. Say something about how when you got your plan, the rep told you to always ask to talk to customer relations whenever you have changes to make as only they can do them or something.