It is going to depend on exactly the patterns you want to match and the format of the changed result. Assuming the first subtitle block's subtitle text is a single line of text and the following subtitle block's subtitle text is a single word of all-lower-case characters (assuming that's what you want from your previous "Grep pattern needed" posting) followed by optional period punctuation mark, the following grep pattern will find that pattern and capture group parts for reassembly into the desired result:
^(\d+\s+\d{2}:\d{2}:\d{2},\d{3} --> )(\d{2}:\d{2}:\d{2},\d{3}\s+)(.+)(\s+\d+\s+\d{2}:\d{2}:\d{2},\d{3} --> )(\d{2}:\d{2}:\d{2},\d{3}\s+)((?-i)^[a-z]+[.]?[\W]*)$
The question is then what to you want the result to look like?
1) Just move the word from subtitle block 51 to the end of subtitle block 50's subtitle text and leave everything else the same?
The replace pattern:
\1\2\3 \6\4\5
will produce as a result:
50
00:04:35,000 --> 00:04:42,840
However, what can happen is this can come at the cost of disregarding the larger whole.
51
00:04:42,840 --> 00:04:43,000
2) Change subtitle block 50's timecode end time to subtitle block 51's timecode end time along with moving subtitle block 51's single word subtitle text and completely remove what remains of the subtitle block 51 entry?
The replace pattern:
\1\2\3 \6
will produce as a result:
50
00:04:35,000 --> 00:04:42,840
However, what can happen is this can come at the cost of disregarding the larger whole.
3) If you chose 2), the subtitle sequence numbers for the remaining subtitle blocks will no longer be sequential. Fixing the remaining subtitle blocks' subtitle sequence numbers so they are sequential can't be done using just grep find and replace patterns. You're going to need something beyond grep like a script to accomplish that and you might as well put all the finding and replacing in that script solution also to keep all the manipulation mechanics in one tidy package.