As far as I can see, the three biggest changes in existing syntax are:

1. Sigils don't change on different access (%foo[1])
2. Arrow is replaced by dot, concatenation is done by underscore
3. For's topic's location changes (for @foos -> $foo) and C-style loops are done with loop

1. Because the sigils don't change, $foo[1] will unambiguously mean $foo.[1] (Perl 5: $foo->[1]). That means that converting existing scripts will be easiest when you simply replace %foo by $foo, instead of the other way around. If I understand correctly, context will force $foo into being a hash reference.
2. First, replace all dots by underscores, adding whitespace if it's not already there. (You may want to rething your design anyway. Perhaps the stringifying unary underscore can help you out: _($foo, $bar, $baz).) Then, simply substitute arrows for dots - or remove them, if you're not calling an object method, because $ref->{foo} will be $ref.{foo} which can be shortened to $ref{foo} (which is element 'foo' of hash %$ref, not hash %ref!)
3. Then, find all for and foreach loops, and change them

1. Backup
2. Automatically convert
3. Try
4. Fix
5. Try again

U28geW91IGNhbiBhbGwgcm90MTMgY
W5kIHBhY2soKS4gQnV0IGRvIHlvdS
ByZWNvZ25pc2UgQmFzZTY0IHdoZW4
geW91IHNlZSBpdD8gIC0tIEp1ZXJk

--
perl -pe "s/\b;([st])/'\1/mg"

I think this is one advantage of the Perl 6 design strategy: every change, every development is public. By the time the design is finished and a Perl 6 interpreter is production-ready, most people will be familiar with the changes. It won't be strange and new, but old and comfortable, as easy as print. I already find myself typing along in Perl 5 and trying to stick in Perl 6 features and syntax. Then I pause, frustrated, and have to "back-translate" (maybe I should write a p62p5 translator :). Others will be infected similarly. And those that aren't will have a large group of more knowledgeable users to help them, just like they do now. Do you remember who first introduced you to the joy of map? Users around the world will find it's exactly the same with given, etc. It's a cool feature they never knew they had before, but now that they know it, they'll use it every chance they get.

"You can do things that change everything without changing anything." -- Brian Ingerson on Perl 6, YAPC::NA::2001

This is a kind of philosophical issue. You probably won't be successful as an IT person (read, computer geek) in the long run if you are not comfortable at learning and even relearning from scratch several times a year. I thought it over many times from the very moment I was presented with a chance to choose this occupation, this way of life. And that perpetual learning, eternal school excites me much :) I find the root of all computer-related fun in this very aspect!

1. a bit of adrenaline is a good thing ;-) (try jumping out of a plane ;-P)
2. i'll improve my programming skills
3. it's a great opportunity to learn something new
4. changes are the best things in the world

Is Perl 6 going to be any more "Perl" (i.e. the same language we've seen evolve from Perl 1 through 5) than say Ruby?