I've never thought of a Flex Plus award as a "Classic". EK use the term for any award ticket and if we relate Classic to the good old days it would be a Saver Reward. This level of award used to be the only one (well before the current fare families existed) until EK realised they could ask for double miles and somebody might pay it.

As to whether it's better to buy an outright award ticket or upgrade an economy ticket, historically it's always been the latter. Nowadays however with the cash price of tickets having gone through the roof, and the different costs (in both upgrade miles (plus taxes) and purchasing cash) for the different fare types it may well be different.