No, if you upgrade today to from HALion Sonic 2 to HALion 5 and activate your HALion 5 license, your HALion 5 license will be eligible for the free grace period update to HALion 6.
If you decide to update after the release of HALion 6, you will have to pay double the price to update from HS2 to H6 compared to HS2 to H5.