I find it rather ~odd~ that you are trying to claim this tense change as in your favor and pushing me on it. Wasn't it I who had been hinting at this for several weeks now? Didn't I finally 'hit' you with this one before you 'took this cause'?
Of course, I have ~already~ explained it to you! You just weren't listening. (*grin*). The verb tense change indicates John is referring to a different 'millennium' which has yet to take place -namely the New Heavens and the New Earth!
This is in fitting with Eph 2, which is so strikingly similar in message to Rev 20 when you allow the words of Rev 20 to speak for themselves rather than throwing ideas of a temporary earthly reign after the 2nd Coming and before the instution of the New Heavens and the New Earth since that concept isn't remotely even hinted at in the text itself.
No, vs 6 states of a future 'age' which in which John tells us that those who have "lived and reigned with Christ a thousand years" are blessed and that they "shall be" priests and kings of God and of Christ and they "shall" reign with him a thousand years.
Look at Eph 2:
5 Even when we were dead in sins, hath quickened us together with Christ, (by grace ye are saved;)
6 And hath raised us up together, and made us sit together in heavenly places in Christ Jesus:
This is strikingly similar to Rev 20:4:
4 And I saw thrones, and they sat upon them, and judgment was given unto them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark upon their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
Eph 2:7 goes on to say:
7 That in the ages to come he might shew the exceeding riches of his grace in his kindness toward us through Christ Jesus.
In the same way we see Rev 20:6 tell us:
6 Blessed and holy is he that hath part in the first resurrection: on such the second death hath no power, but they shall be priests of God and of Christ, and shall reign with him a thousand years.
Jean