The present kernel 2.6.35 as with all previous releases is upgradable however to my knowledge a kernel downgrade is not possible and even if it were kernel 2.6.27 is now well outdated. For example to run Ubuntu 10.04 off kernel 2.6.27 would be almost if not impossible to achieve
However if you are prepared to run an unsupported distro. I thoroughly recommend Linux Mint 6 Main Edition which is built upon Ubuntu 8.10 Its easy to install and easy to use plus it comes with much of the software you are likely to need preinstalled.
Linux Mint 6 as does Ubuntu 8.10 runs off kernel 2.6.27 as default
https://wiki.ubuntu.com/IntrepidIbex/TechnicalOverview
Linux Mint 6 Download
http://www.linuxmint.com/edition.php?id=31
The Perfect Desktop - Linux Mint 6 (Felicia)
http://www.howtoforge.com/the-perfect-desktop-linux-mint-6-felicia
Linux Mint 6 User Guide pdf.
http://www.linuxmint.com/rel_felicia.php
You download the ISO. image of Linux Mint 6 then you need to create a Bootable LiveCD for installation
Linux Mint 6 can also be run direct from the LiveCD from Booting up without touching your Hard Drive
Certain enhancements can be made such as upgrading the Nvidia Driver from 180 to 195 and upgrading to Firefox 3.6.10 using Ubuntuzilla
http://sourceforge.net/apps/mediawiki/ubuntuzilla/index.php?title=Main_Page
HowTo Install Nvidia 185, 190 and 195 Drivers on Ubuntu
http://www.webupd8.org/2009/08/how-to-install-nvidia-190xx-drivers-in.html
Running an unsupported distro. although not ideal is not as problematical as it may initially sound. I still have a small partition running Linux Mint 3.0 which is built upon Ubuntu 7.04 and runs off kernel 2.6.20
LUg.