I have 2 guesses for why you cannot keep your current plan.
1.) The plan you have is not available anymore and when you change devices or any plan changes, you can no longer hold on to grandfathered plans

2.) Is what I call the iPhone effect LOL Some phones have their own list of compatible plans and options. This does suck and is a way for cellphone companies to get you on a newer plan so they can apply new rules to you.

I could be wrong but those would be my guesses, I hope I helped. Let me know what you think

hoping you can share some advise - do you think I could score a better plan than the one i have below?( old sprint contract grandfathered in) Sprint wants me to switch to a simply everything plan plus $10 premium if i want an Evo 4G -- why the hell can't i just keep my plan and add the $10 premium? Thanks in advance.

Free & Clear Family 2000
Monthly charge for service plan $85.00
Plan Includes
•Sprint Free & Clear Plan
•$85 Sprint Free & Clear Plan
•Minutes Are Shared On This Plan
•All Minutes Include Long Distance
•2000 Anytime Minutes
•Unlimited Sprint To Sprint Calling
•Caller Id, Call Waiting
•Three-Way Calling
•Voicemail
Plan add-ons
• America Roaming Included $0.00
• Sprint Unlimited Data Pack $0.00
• Unlimited Night & Wknd Min 7pm $0.00
• Cellular Call Detail $0.00
• Sprint Casual Data $0.00
• Unlimited Text Messaging $0.00
• Picture Mail $0.00
• Total Equipment Protection $7.00
second phone on the line is BASIC no data plan or text messaging