"once seen as" is correct and usable in written English.
This phrase is used to express a change in opinion. For example, "Once seen as a fringe movement, veganism has now grown in popularity and acceptance."
He was once seen as a potential mayor of DeRidder.
"It was once seen as a primer for black power".
A failure to repay debts was once seen as a sign of moral laxity.
Each was once seen as magical technology that would transform children's education.
However, once seen as a British success story, the North Sea now looks anything but.
But then, banning smoking in restaurants, workplaces and bars was once seen as crazy, too.
Once seen as an exotic combination, sweet-savoury pairings are now commonplace and very enticing.
Once seen as a lender of last resort, it now acts pre-emptively.
A vote for the Lib Dems was once seen as wasted.
What was once seen as evidence of corporate fitness for the moment looks like anorexia.
Satan, once seen as a possible right wing for Sidney Crosby, had his playing time shrink.
