Change user permissions programmatically on the fly

I have in the code I don’t conrol such a line which returns false:

if (!$user->authorise('core.create', 'com_jdownloads.category.'.$option->value)) {

I can run my code before and after this line (via a plugin).

I need to temporary allow user to permit the action currently banned with the code above, and after done, and change permissions back.

Is this possible and how?

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Long story upon @Irata request

@Irata I’m a developer of extension, which sends email notifications on Joomla content update – NotificationAry (NA),
https://extensions.joomla.org/extension/notificationary/

Though it mainly works with JDownloads, I meet a problem reported by a user. NA allows users to subscribe per category and in User profile one can check categories to be notified on changes in.
Screenshot:
http://images.my-dev.org/x/2020_07_18_16_32_52_nn.png

This works in backend, but fail at frontend.
http://images.my-dev.org/x/2020_07_18_16_34_51_vq.png

To generate a list of JDownload categories I use your field
/administrator/components/com_jdownloads/models/fields/jdcategoryselect.php

In my code it looks like this:
https://github.com/gruz/GJFields/blob/master/categoryext.php#L196
But at frontend it always returns empty array of options. At least now. It was ok some time ago, but maybe years ago.

I checked JDownloads code and it doesn’t allow registered users to get categories list
administrator/components/com_jdownloads/models/fields/jdcategoryselect.php
Line about 115

if (!$user->authorise('core.create', 'com_jdownloads.category.'.$option->value)) {

So at Frontend a registered user is not able to get the list of categories.

I want to get the list of your categories and to use native JDownloads field for this. I try to avoid copying other extensions code into mine because updates can break things. I had to copy JDownloads code, https://github.com/gruz/GJFields/blob/master/categoryext.php#L199 to make it work dirty