Safe to upgrade from 1.5.6 to 2.x.y?

Your question is very subjective. What are your particular safety concerns?

I would not have released 2.x if I felt it was unsafe to use. However, the complex impossible issue of universal compatibility is addressed differently in 2.x. Non-standard bundles may require configuring Loco Translate to understand them. This could result in a non-trivial upgrade for you. I can’t elaborate on that, because I don’t know anything about your site or how well it’s been localised.

The bottom line is that I’d never install (or upgrade) ANY plugin on a live server without first testing it in a development environment. My suggestion is you get hands-on experience safely before you upgrade your site.

specifically,
– will current data survive?
– is old data converted to new standards (if applicable)?

perhaps the first question should be;
– does 2.x.y use a different data format than 1.5.6 does?

Based on your answer, we’ll advise our client a thorough investigation, instead of an “acceptable risk update”.

Thank you for your time!

will current data survive?

The data (meaning your translations) are held in PO/MO files. These will NOT be deleted by the plugin when you update. The plugin in fact never deletes any files unless you specifically ask it to.

is old data converted to new standards (if applicable)?

Translations already covered – so no. However, Loco Translate preferences (saved as WordPress site options) will be converted. This is unlikely to cause any issues.

does 2.x.y use a different data format than 1.5.6 does?

Only plugin preferences. All translation data uses Gettext standard formats and is all held in regular files on disk.

Just to clarify, no translation data is held in the database, nor is any of it proprietary to Loco Translate. You can switch between PO editing software as you wish.

excellent news, that is ??

I will keep a record of the current settings, update the plugin and correct settings if needed.

Thank you for your time and effort!