since 2005 (or so it says on the bill) they had issued a new MX$50 bill made of polymar plastic. (similar to Australian money) Still the same potrait of Jose Maria Morelos at right, same size, and the insinga or coats of arm (cross cannons) at center. Differences are the back of the new one is an image of El Acueducto de Morelia w/ butterflies in foreground whereas the old one an image an anonymous butterfly fisherman on lago Patzcuaro w/ his co-worker floating in the background. So the notes are very similar in design and are still worth the same as well as the fonts. So are the old ones still legal tender or would they need to be traded for the new ones?

They're still very much in use, and likely will be for a long time. The blue plastic 20-peso note has been in use for a few years, yet I've seen the occasional paper 20-peso note still in circulation.

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<hr>The blue plastic 20-peso note has been in use for a few years, yet I've seen the occasional paper 20-peso note still in circulation. <hr></blockquote>
Well the MX$20 notes are still exactly the same design only on different material whereas the MX$50 looks different on the back and even the colour is a little different. Overall, still look completely different between new and old. The older ones are like a burgundy/violet colour whereas the new ones are more pinkish colour and the fonts are different too. In other parts of the world like Great Britain a redesign like this means the older ones are no longer good like the £20 note in which the older ones have a potrait of Faraday whereas the current ones have the potrait of William Elgar at back. Everything else like size, queen's potrait, and colour combinations are the same. Perhaps that's not the case in Mexico.
In other words a complete design could mean the older ones are no longer good whereas if the designs are the same except all few new anti-counterfeiting features added on, which is part of the reason to go plastic in this case. (The other is longer circulation life)

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<hr>In other words a complete design could mean the older ones are no longer good <hr></blockquote>In this case, no.
