What happens if I sell my shares before I receive the new shares resulting from the Rights Issue/Open Offer?
If you do sell your holding before you receive the new shares, you will have to make two sales, both of which would incur a dealing commission.
After the shares are added to your portfolio and you are certain that they are counted as one holding, you can sell them. Beware however, that sometimes the new shares are issued as Registered (or REGD). This means that they are somehow different from normal shares. They might not for example, be entitled to the next Dividend. After the event has passed, they will merge with the existing shares and will count as one holding. However, until this time, they are technically two different shares and will require two separate sales, incurring two lots of commission.