Yes, you may upgrade, or increase your software user level, and pay only the difference plus the pro-rated renewal price to bring your software up to date if Customer-Hosted (Hosted locally on a server that you provide). For example, say you bought the 50 user edition 6 months ago for $500 and wanted to upgrade to the 100 user edition, it would cost you $562.50. $500 for the upgrade ($1000 [100 user cost] – $500 [50 user cost]) and $62.50 (125 * 6/12) to renew your existing 50 user edition. The new expiration date would be set at the latest purchase date.
However if you are EIOBoard-Hosted instead (we host for you on our EIOBoard.com server), then you pay the difference between the levels you are currently at, and the new one that you need to move into. For example, say you bought the 50 user level 6 months ago for $500 and wanted to upgrade to the 100 user level, it would cost you $750. $1000 for the upgrade ($1000 [100 user cost] – $250 (Credit remaining from the 50 user level ($500 [50 user cost]) that had been paid.) The new expiration date would be set 1 year from when the upgrade payment is made.