It's time+simple past tense

It is time you shift
uses the subjunctive shift, as with require/demand verbs:
It is required that you shift
which has the (mainly) BrE equivalent of
It is required that you should shift
(see Swan, Practical English Usage, Subjunctive, Should)

I think your version could be also valid, but suggesting less of a mandatory/presssing requirement and more of a doubt.

Both versions are used at Google, with yours being more frequent (and present on a  university site here):

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Perhaps it's time you shift your focus off of yourself and
this incident which most rescuers/volunteers encounter and onto the
greater good of a difficult cause.

[link]
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If you love what you do, if your business lights you up, you wish you
could attract more clients--but you're confused about marketing and
you just hate to sell, then it's time you shifted the way you think
about getting clients...

[link]
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Calive wrote:

This is the headline from the brochure of a leading car maker: Isn't it time you shift to a smarter drive? In my opinion, the correct sentence would be: Isn't it time you shifted to a smarter drive? Am I right?

Well, there are several combinations

• Isn't it time to [do something] = I wonder if it is time to [do something]
• Isn't it time you do/go/… = A way to say someone that it is about time for him to do something, either to remind him or to order him or…
• Isn't it time you did/went/… = You missed something but now you ask if that was about time for you to have done it at all
• Isn't it time you did/went/… = Isn't it time when you did/went/… you ask if some past event had happened simultaneously with another past event
• Isn't it time you did/went/… = You express a desire something to happen though it looks it is not going to (Isn't it time he/you resigned?, Isn't it time he/you gave up?)

You can use other pronouns not only you, for example we, but with he/she/it/they the sentence has to be carefully tailored.

Not to forget the other usages

• Isn’t it time we do not have? (we do not have time)
• Isn’t it time that this program is going to squander far beyond our expectations?
• Isn’t it time for out meeting?
• Isn’t it time?

• Isn't it time you shift to a smarter drive? a reminder or request
• Isn't it time you shifted to a smarter drive? a missed event, desire or reference to another past event

Welkins2139 wrote:

It seems to me the difference between 'shift' and 'shifted' is a degree/how much  that a car company wants you to shift.

Yes, that is the commercial/marketing point of the different phrasing.

Thanks for your replies. I found something on this subject

by BBC. It explains the rather strange use of simple past tense.