Hello,
I read and search through this wonderful forum and tried different approaches but it seems I lack some knowledge and neurones ^^

Here is what I'm trying to achieve :

file1:
test filea 3495;
test fileb 4578;
test filec 7689;
test filey 9978;
test filez 12300;

file2:
test filea 3495;
test filed 4578;
test filec 7689;
test filex 8978;

results:
test filea 3495;
test filed 4578;
test filec 7689;
test filex 8978;
test filey 9978;
test filez 12300;

comparison in based on last field (field $3), new content from file2 (here content with "key" 8978 is new) should be added to final output and content that is different in file2 (test filed 4578; here) should replace file1 one.

here is where I am now:

awk 'NF { key=$NF;keys[key]++ } NR == FNR { key1[key] = $NF ORS;rec1[key] = $0 ORS;next } { key2[key] = $NF ORS;rec2[key] = $0 ORS;next } END { for (k in keys) { if (key1[k] == key2[k]) { print rec2[k] } else { print rec1[k] } } }' $file1 $file2 > $file1.updated

for readability:

awk '
NF
{
key=$NF;
keys[key]++
}
NR == FNR
{
key1[key] = $NF ORS;
rec1[key] = $0 ORS;
next
}
{
key2[key] = $NF ORS;
rec2[key] = $0 ORS;
next
}
END
{
for (k in keys)
{
if (key1[k] == key2[k])
{
print rec2[k]
}
else
{
print rec1[k]
}
}
}'
$file1 $file2 > $file1.updated

but.. this doesn't work well :/