I have a half used CRTW ticket, with a HKG-SIN-BOM-LHR segment left. I am not finding availability on the days I want HKG-SIN-BOM (one or other sectors waitlisted with SQ). The ticket is expiring in a few weeks which gives me very little room to juggle with dates.
If I decide not to fly HKG-SIN-BOM, can I still use the BOM-LHR leg of ticket to salvage something out of it? Or do I have to get it reissued with HKG-SIN-BOM as surface to use the last leg?

There's a rule (isn't there always?!) that does say travel must be in sequence of ticket issue, so technically they can insist you have it reissued.
However, if they're different carriers involved for your BOM-LHR than those other segments, I'd try ripping out the unused coupons* (leaving the BOM-LHR intact) and then winging it at check-in at BOM. You can always plead ignorance if they pick up on it, however if they are different carriers from your inbound to BOM to your outbound, then it is unlikely to be picked up IMHO.
*Provided it's a paper ticket and not an E-Ticket - if it's an E, then you'll more than likely get caught as they'll be able to see the unused segments on the E-Ticket still.

Have you tried UA to get your HKG-SIN sector confirmed as they also fly the route - may get you out of a hole if you can get them and then SQ for the SIN-BOM.

They are seperate carriers. In the worst case, I will try winging it out. They are paper tkts.
I can try UA, but they misconnect to SQ flight to BOM, so I will have to overnite at SIN, which might be an issue as I am juggling for time at the moment . And since I am changing carriers (UA instead of SQ) I will have to reissue the tkt then anyway.

Are you sure about this? As long as it's the same route, such a carrier change might be the same as a simple date change -- i.e., rebooking rather than reissue.

cheers,

Henry

There is another theread here , discussing this issue. Looked like changing from SQ to TG meant it was a re-issue. This was e-tix though. With paper ticket, it should be possible to do without a re-issue.

OTOH, to the OP, If you do have to re-issue, Why not go HKG-BKK-BOM (TG), or HKG-ICN-BOM (OZ). but this definitely means a ticket re-issue and re-routing.

UA claims they will charge USD 125 as the original routing is not on a UA flight/codeshare.
HKG-ICN-BOM won't work as OZ flies ICN-DEL, not daily tho'.

On all my paper RTW tickets so far, it was clearly endorsed as 'Valid on *A carriers only". Now, if there is no routing change, you should be able to change the carrier without having to pay for it. though i have never done that, i am sure others here can chip in their experiences.

I would ask the original issuer of the ticket to make the change. And if it was UA, then may be ask the original office which issued the ticket.

Aha.!! thought OZ had a flight to BOM. So, if you do have to re-issue, then i guess HKG-BKK-BOM is the only other option not going via SIN.

I had the same issue on a ST RTW - I had to break my RTW to return home for some family business, and joined my RTW in mid-journey. The route was supposed to be:

PEK-NRT-KIX
KIX-AMS-BCN
BCN-FCO-TLV

but since I had to go home, it became:

PEK-AMS-TLV - this was originally a R/T, but I made a "stopover" in AMS to rejoin my RTW...
TLV-AMS-BCN
BCN-FCO-TLV

I lost the AMS-PEK part of the R/T

I tried being "honest" about it, but KLM actually told me to cancel the KIX-AMS portion and to rip up the coupons PEK-NRT, NRT-KIX, KIX-AMS.