Really, honestly, trying to upgrade an existing 1.5.x store to 1.9.x is just stupid and broken. write a "real" upgrade script, one that has some meaningful comments, and doesn't result in 404 for /downloader/* urls. Are you seriously saying that I should install a new 2.x version and then try to use the hopelessly crappy, and broken import tools to get the data from my 1.5.x store imported?
Magento, at one time, was the bees-knees, now it seems like a mostly-broken pile of sh*t.
Yeah, learned through pain and tears, this is the upgrade path
22.214.171.124 -> 126.96.36.199->188.8.131.52->184.108.40.206->220.127.116.11-1.9.2.x
Up till 18.104.22.168, all upgrades are fraught with major issues which are made even worse if you decide to use one of the intermediate versions. And 22.214.171.124 should have been 126.96.36.199 as the major changes in it are extensive and totally incompatible with the 1.4 branch. For the first time in my life, 188.8.131.52->184.108.40.206 actually seemed to make it through without a database upgrade script pukefest.
And then there's the 1.9.2.x Migration to 2.x which will make everything you've already experienced seem like an amazing dream.
Upgrades are best done on a staging/dev server until you find the right combination that allows you to do a multi-stage git branch commit and in the pre-1.9 versions, you might be doing the final database conversion off-server before uploading and importing it by itself into the live server, despite having a version control system like git in place.
If you have problem about upgrade old version to new version, why do you seek help to update online store. Magento 1.9 and Magento 2 are good. And it is better to upgrade Magento 1.5 to 1.9 or Magento 1.5 to 2.0
If you don't try, you will never know the result. I believe that some problems you have can be solved if you share it. or you can try upgrade to magento 2, http://tinyurl.com/magcomupgrade2