They would have known what would happen, even before said Eastern expansion. And I very much suspect that many were in favour.Look, EU freedom of movement was never meant to enable gangmaster led zombie eastern European work forces destroying minimum labour prices across Europe.
It was designed to enable companies to set up across Europe and allow their staff to work from different countries, or for Germans to work in France or vice versa. Adding skillsets and growing economies. Not shifting the lowest common denominator or allowing people from countries with a low gdp to come earn from a higher gdp country and send the money back home (economic withdrawal) and abuse higher levels of benefits from countries with a higher cost of living.
The problem was the EU expanded maybe too much too quickly without changing the tenants of freedom of movement which was not intended to be exploited in this way.
Cheap migrant labour (and the resulting depressed wages but *also* cheap food, etc) were probably adjudged to be good things at the time, by the leaders of Germany, France, and probably also the UK.
They will have commissioned and read numerous reports on the possible economic consequences.