When you change plans during the middle of the month, yes the next month's bill is prorated to account for that partial month. Depending upon whether you upgrade or downgrade plans, that prorated bill will be lower or higher than it will be on subsequent months. If you login to your account, you should be able to see a detailed breakdown of the charges and don;t forget that the modem lease fee is a separate fee from quoted plan prices.
If still uncertain, and email to one of the ViaSat corporate employees here at email@example.com should yields some clarification of charges. You need to include your account and contact information with that email (do not post that info here since this is a public forum). It's the weekend, so don't expect an immediate reply and you may need to followup on Monday AM.
P.S. Yes, both Jim and I are real people - just customers like you ;)