Which license do you currently have? Check the ALM (Automation License Manager) software under the "product" column. You may even have other licenses that would require upgrading. The COMBO license allows the use of the classic Step7 and Step7 in TIA Portal. 

For TIA Portal's Step7 Professional, use 6ES7822-1AA23-0YE5. This is to upgrade to V19. That would give you the freedom of using V18 as you plan, but also to use V19 later on.

Take my advise at your own risk. Consult with your local distributor to reduce your risk.

Hi,

also please check this FAQ - V19 License also works with older TIA Versions and price is similar:

Which license do you need for TIA Portal V19 if you upgrade from an existing STEP 7 version V5.x, V11 .. V18 to TIA Portal V19?

Regards,

Towome