how to change repo in lubuntu

foreword:
i own an old laptop, 5 years ago i installed lubuntu 15.04 ,
now i've had to reinstall it but the repositories don't work anymore.
the laptop is too old for lubuntu 15.10 .

now the three questions:
is it possible to go on using lubuntu 15.04 and to use repositories of another version?
is it possible to go on using lubuntu 15.04 and to use repositories of another distro (debian, for instance)?
if it is possible, how should i do that?

thanks for any reply.
pb