I wonder what [WHICH] of the following is more
Can you please change my email
address? Could you please change my
email address? The second one sounds
as if the request is more urgent, to
me. So I would take the first. But I
heard the "could", "should", etc.
forms are more polite forms of "can",
"shall", etc. What's the underlying
reason and is this true at all?
The underlying reason for the differing levels of politeness in modal use is how they developed historically.
Hundreds of years ago, modal verbs had tense. There were present tense forms and there were past tense forms. Such is no longer the case. In modern English, modal verbs are tenseless. This allows them to operate in all time situations, past, present and future.
This does not mean that each and every modal verb can do each and every modal function. They are still somewhat constrained by their historical roots and the meanings they hold today.
Historical present tense form - Historical past tense form
can - could
will - would
may - might
shall - should
In English, the past tense FORM of both lexical verbs [jump, run, write, etc] and the Historical past tense FORM of modal verbs are used to perform a number of different tasks: politeness/deference/doubt/non- real or counterfactual/... .
"I was wondering ..." is more polite/deferentialsofter than "I'm wondering ... "
"Did you want something to eat?" is more polite/deferential/softer than "Do you want something to eat?"
It's vitally important to remember that this use is only a use of the past tense FORM, it is NOT a true past tense, in the sense of time, use!!
Another important thing to remember is that when it comes to the modals, it is the HISTORICAL past tense FORMS which are used, again, to effect a greater sense of politeness, deference/softness/... .
Even the Historical present tense FORMS are polite but they are not as polite as the HISTORICAL past tense FORMS because they are part of this grammatical "English uses past tense FORMS to effect greater politeness/etc" routine.
In your examples, there is no difference in meaning between 'can' and 'could'. In this sense both hold the meaning of "Is it possible for you to change my email address?"
There is a difference in the level of politeness, which I hope I have explained satisfactorily.