I take it your pages are currently .asp and will be changed to .php ?
If you 301 rediect all the old the the new, there may be a short term drop, but over the slightly longer term it shouldn't cause any issues.
of course this depends on what other changes you will be making if any.
also, I suppose you could switch to php but keep your .asp page extensions the same, so there would in effect be no noticable difference
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