Assuming they’ll want to, will the post-Trump administration be able to restore the policies (i.e. environmental) as quickly and easily as those policies were taken apart?
First, this question might’ve been better to post in GQ, as it does have a [number of] factual answer[s]. (ETA: you could report your post and ask a moderator to move it to GQ, and avoid some perhaps sarcastic future replies. Just a suggestion, I am not a SD moderator.)
Second, the simple answer is NO, restoring previous policies will require actions similar to those that were required to set the policies in the first place. In the case of general policies that were originally established or dismantled via Executive Order, then a new EO might do it. However, many federal policies are actually rules and priorities set within the various executive departments and agencies. Restoring things like environmental safeguards and financial regulations which have been scrapped by the current administration will take investigation, validation, written establishment and propagation within each federal entity, driven by the new appointed heads of those entities. That takes a lot longer and requires more effort (including painstaking study/justification if we’re going to be duly diligent) than does careless dismantling.
How quickly a new administration could “restore policies” depends greatly on how those policies were repealed. The easiest would be those done through executive orders or other administrative directives. An executive order is a directive from the President to the executive branch on how to enforce a law, and can be reversed or changed pretty much as quickly as it takes the new President to draft up and sign a new one. The new Administration should take some time to thoroughly vet the Executive Order, lest you end up with the three iterations of Trump’s Muslim ban. Also, cabinet secretaries and agency heads have some discretion in how they administer and enforce the laws under their purview that can be changed relatively quickly.
As xenophon41 notes, changing federal rules is a more laborious endeavor that requires the agency to go through a formal process that involves analysis of costs/benefits, posting for public comment, etc. New cabinet secretaries could begin the process pretty quickly, but depending on the complexity of the rule it could take many months before an agency could enact a new (or restore an old) rule.
Importantly, however, most of the rules repeals that were effected early in Trump’s term were done through the Congressional Review Act. This allows Congress to invalidate, through a joint resolution subject to Presidential veto, a rule within sixty legislative days of its enactment. The law also states that the agency cannot reissue the same rule or a substantially similar rule unless specifically authorized to do so by new legislation. Fourteen Obama-era rules were repealed through this procedure during the first five months of Trump’s term. So you would need an act of Congress to restore those rules, subject to all the challenges that presents.
There’s also a difference between undoing a policy, and undoing the results of that policy. Take the EPA, for instance: It’s a policy change that the EPA no longer considers pollution to be within its purview. So big industries will now start polluting like crazy. Even if, in 2021, the new EPA leadership decides to start doing their job again, there will still be those years worth of pollution in the environment, and it’s going to take a lot longer to get them back out.