What can persuade you to hire Junior Ruby devs with significant PHP experience?

Hi!

I am writing to ask for your advice.

Currently we have several software developers with 7+ years in PHP
that have strong desire to switch to Ruby on Rails.
They have passed a brainbench certification in Ruby on Rails with
excellent results, but have only several months of experience in Ruby.

Are there any possibilities to find Ruby project for them? What they
should do to persuade a client that they will be good at Ruby?
Could free piloting project may be a solution?

Are there any other things they can do to prove their capability to
fulfill Ruby projects?

What do you think?

Thank you in advance!