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

Both versions 15.04 and 15.10 went End Of Life back in 2016.

No, you can not use recent repositories with an older version with Ubuntu or any debian derivative.

Post the make/model of your laptop with its specifications and hopefully we can suggest a supported distribution that will work on your computer.

No, in both cases as those "other version"s are not supported anymore too.
The oldest "still supported" ubuntu (and thus lubuntu too) version is 16.04 (Xenial Xerus, a LTS release) and it won't be supported very long anymore as end of standard support is in a year.
All NON-lts releases (like 14,10, 15.04, 15.10 etc) have a lifetime of only 9 months.
Debian "stable" releases have a normal support of about 3 years and then another 2 years of LTS (critical patches and security fixes only).
Debian 8 (from 2015) ends life next june
Debian 9 is from 2017, so very much newer then your lubuntu
and I'm not even talking about Debian 10

PS: ubuntu LTS releases are based on SID, the UNstable version of Debian.
Your lubuntu was based on Debian Jessie (8), which is almost out of support too as shown above.

i am not so bright, i don't even understand what end of support implies.

i just meant to keep my old version and getting some packages from new ones (or different ones).
you told me it's not possible.
thank you for the answer, even if ..well bad news...

by the way, atom 1.6 GHz, 1G RAM

End of support, sometimes referred to as "end of life" (EOL), means the version will no longer receive software and, more important, security updates. The latter will render your install vulnerable to hackers and malware.

Your best course of action would be to back up your data and install a supported version.

Newer versions of a distributions generally use newer kernels and software that are not compatible with the older distribution/software which is why it is not possible.

Atom 1.6 GHz, 1G RAM at first glance sounds like a N270 which is a 32 bit processor. You might like Bodhi or antix distributions.

nope you can't change repo's on a super older distro. who said the computer is "too old"? is it lacking sufficient memory? or....?

No, it got a 32-bit cpu and a lot of the "newer" distribution releases are 64-bit only.
I originally thought CentOS 6 might be something for him, but its support ends in december:
My Slackware still fully supports 32-bit and will be in the near future (the to be released 15.0 version).

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yep. been using x64 for so long that totally slipped my mind.

You can do a complete reinstall with Debian, if you need a x86 (32 bit) system.
First check that with The line
is relevant. Here is an example for a x86 only system.
The line Architecture refers to the arch of you installed system, so it can be i386 only, but the CPU may support AMD64 (also on Intel) too.

There are also Atom's with Intel 64 support, so check that first.

Whether it's 64bit capable or not, resources are very limited. These two distros surely are a good recommendation for that.