In order to upgrade you will first need to renew your existing Extension Pro Pack license. After you have renewed, you will then be able to upgrade to the Pro Pack paying only the difference in price of the license you are upgrading to the license you are upgrading to. Since you plan on upgrading to the Pro Pack, you would not need to renew the Slim Plus template since it is included in the Pro Pack. I hope this make sense.
Let me know if you have any further questions.
ps. I ‘ve not done the math, but it might be cheaper for you to let both those items expired and purchase the Pro Pack instead of renew then upgrade.