Updating older concrete5 site
To go any further, you need to rebuild in v8.latest. Rebuilding can be assisted by content migration, but is also hindered in that addons don't map across 1:1. For most addons there are equivalents in v8, but not direct equivalents for data.
A lot depends on what you are doing with the site. If it is performing well and meets your needs, then update to 5.6.4 and keep going (many people drive trucks 20 years old). If you have major new content areas planned, then it may be worth rebuilding in v8. If you don't have many pages, rebuilding in v8 is not overwhelming. If you are hosting a large online community, rebuilding in v8 can become a considerable project.