No-one's been on here for a long time, but FWIW, here's my take (as an inveterate re-writer of others' [& Trad.] songs) on it all.
By all means, alter, as long as you acknowledge [not doing is "passing off"].

Before getting as far as Jeri's re-post by Sarah Morgan, I was sure it was Sarah who told us that the "Overseas in India" version of "Home Lads, Home" had appeared in a magazine [I was unaware how recent], and I see that's confirmed here.
Strikes me that this kind of quasi-legitimately anonymous output almost certainly has an earlier printed source, but post CFS's oeuvre. In publishing of all ages, esp. pre-internet, acknowledging (c) was patchy. An editor, receiving (or finding) this with no authorship, could easily explain the convincing source, just by reading the poem. It might even accompany that mooted earlier non-CFS "original". Believing it anonymous (must have been tons of amateur War poetry written) why would our editor have sought further confirmation?
And has anyone suggested that, as a working poet (and/or being paid per line), CFS herself might have sent it to an Indian-leaning publication with tweaks to suit their audience? (Handel re-used his own tunes thus: efficient use of his creativity). The "British soldier" element might have been added alongside [by CFS?], in the manner of "All the facts in this story are true" banners in film credits. It's good marketing! She may have sent it elsewhere, too, but the others not been published, thus several localized variants may have existed, but been lost? Mags publishing such material prob. received a surfeit of it, from the Grishkas of this world, but Grishkas who DIDN'T recognize they "were not Schubert" :-), so CFS prob. had little clout (or reputation) which might make an editor choose her work over another submission.

The words themselves: if you've never heard / seen the original, they can make sense. The 'person' in India (someone who experienced the trenches, but returned to their war-decimated population afterwards) is thinking / dreaming about Hampshire, WHILE HE'S IN INDIA. In performance, hearers get little enough time to think about it, but it doesn't 'jar' as being wrong. But "India" could be anywhere non-UK [or "Overseas" doesn't make sense], as long as it has a military, to create the sounds that prompt the reverie. They don't have to be the actual WWI sounds. Of course, it makes MORE sense if it's Flanders, but to a 'cold' hearer, it's not critical.

While some in this thread invoke some good ol' classical names, let me add more, germane to the OP, though not about lyrics. Someone raised the fact with Brahms that Wagner had "stolen" one of his tunes. "Mine's still there", Brahms is supposed to have replied.