sure you can switch your SIM card. SIM cards work across all phones ... some may not work with 4G networks if it's a really old SIM but you seem to have a newer SIM so you can switch into a Sensation and have the same plan.
and yes, you will get charged an upgrade fee unless you're within the buyer's remorse period for your Sidekick. i think it's 30 days from purchase or might be 14 days from purchase.
but who knows .. if you complain and have a legitimate gripe about it they may just swap it out.
if you already own the sensation then yes!
if you want to give back your sidekick for a sensation you have 14 days from date of purchace to return the device. (buyers remorse)
if your sidekick has the data plan then yes it will work just fine since they are both 4g devices on the same network just pop the sim card in wait about 10 minutes for it to start working correctly and then go to your "my tmbile" and change your device to your sensation
(as i remeber when you first log on click the line it shows a picture of your phone and just click this is not my phone and then select the sensation and it should just change the phone)
this shouldnt cost you anymore money like Deacon said f you already own the device.