Makes sense to me.
As a final note, forcing replacement of opponents shield is akin to giving all your creatures momentum for 1 turn as well (since there is no shield to stop them anymore). This is an important point to consider as well... though it is not as bad as permanent mass immortality, it will still have a big bearing on cost considerations...
I'm not sure where to start at for balance there, but some thing to help you out:
-Deflag costs 3
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, 2
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upgraded
-Seraph's ability costs 1
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... this would affect about 5 creatures on average (most players will get 3 to 7 in play for the average deck and game length) ... so thats about 5
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That would give about 7 to 8
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.
The immaterial is dispellable, however, so that would lead to a cost reduction... I'm not sure how much though.
So 5
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seems reasonable enough for the unupped version, though it is probably slightly on the low side for the upped version.
Long story short though, this card could be pretty potent and I'm not sure how big of an impact it will have on metagame.
It would be good to:
1) get an opinion from an experienced PvP'er (or two or three if possible)
2) see if you can get someone to play test it with / for you.
I really do like this card. It adds some fun possibilities and could lead to some interesting strategy behind its application.
I wish you the best of luck with it in the polls.