The phrase "as first believed" is correct and usable in written English.
It can be used to indicate that something was initially thought or assumed to be true, but may have changed or been proven otherwise.
Example: "The results of the experiment were different from what we had expected, as first believed."
Alternatives: "as initially thought" or "as originally believed".
And the investment was not as extensive as first believed.
Would Shanahan have suspended Hagelin for only one or two games had he known Alfredsson's injury wasn't as severe as first believed?
A report this month from the Environmental Protection Agency suggested that the Newtown spill may be twice as large as first believed — some 30 million gallons, nearly three times the size of the Alaska spill.
Investors took comfort from suggestions that new rules on capital requirements, expected to be agreed by European regulators and bank governors over the weekend if all goes according to plan, may not be as tough as first believed.
And the reserved judgment from most members seemed to be based on the potential that what Trump shared with Russia wasn't as secret as first believed.
Since then, researchers have found that it's not quite as absolute as at first believed.
It's worked pretty well.' It is perhaps testament to O'Connor's popularity and pragmatism that the 2000 presidential election has not marred her legacy among more liberal-minded folk as some at first believed it would.
I fell for the "illusion of explanatory depth," but I see now that the positions are not as cut-and-dried as we might, at first, believe.
Thus, the former volcanic cones were not blown into the air, as was first believed, but sank out of sight, the top of the volcano collapsing as a large volume of magma was removed from the underlying reservoir.
Her accounts of the discovery varied; she said she might have handled it in 2011 without realizing what it was, but in 2014 she ascertained that it was not, as she first believed, merely a rough draft of "Mockingbird".
The characters discover themselves not through the books they have read (as Omar first believes) or the places they have been (as the title would suggest) but through Cameron's precisely rendered conversations.
