There is no data loss during a sim card switch. Your device will be provisioned according to the new sim card, though, so if there is no data plan on the account on the sim card then you won't have an Internet Browser or Blackberry Browser. Only what's stored on the sim will be lost. Everything else on the BB stays. You shouldn't be saving contacts and messages on the sim anyway. The BB's built in contact list is much better and it doesn't hold that many sms messages.
Agreed about the BB's contact list being much better. I don't save to the card. Never have never will. I'm not sure I under what you mean when you say "if there is no data plan on the account on the sim card then you won't have an Internet Browser or Blackberry Browser". Aren't the browsers stored on the device itself? I've used my browser through wifi without a sim card in my phone a couple of times...
Are you just replacing your old sim card with a new one with the same phone number, etc.? If so, that doesn't change any provisioning so you should have everything you have now. No data is stored on the sim card so nothing is changed on the device, data-wise. The sim card just tells the phone what services you pay for. That's all.
Prepaid sims don't usually have data plans so you won't have an Internet Browse or Blackberry Browser. You will still have your wifi browser though. I'm just saying with a different sim card you will have a few different options available. Nothing is lost, though.
I want to get a new blackberry 8110. I have the 8100. My pearl line isn't due for upgrade just yet but the other phone is. I plan to keep the old phone and use the new blackberry in place of my 8100 by switching the sim card of the 8100 to the 8110 I mentioned it to the cust serv rep and she said I could but can I?