CitectSCADA Version 6.00 will not require you to upgrade your projects to V5.50 before installing V6.10, however the recommended upgrade path from 5.21 to v6.10 is as follows:

5.21 to 5.31 (Trend system change)

5.31 to 5.42 SPK A

5.42 to 5.50 SPK C

5.50 to 6.10 SPK A (Graphics change)

You should also be aware that customers attempting to upgrade from pre V5.00 will have difficulties upgrading directly to V6.10 as the graphics will not be recognised. In this case it is advisable that the customer upgrade to v5.00 (following the advice in existing KBs) then to 5.21 and so on up to v6.10.

Further information regarding upgrading to v6.10 is available in the Upgrade documentation supplied with v6.10