How to change pattern from a cardigan to Waistcoat

I have started knitting the cardigan using Millamia Naturally Soft Merino Peacock colour. Unfortunately I just realised that I need at least 2 balls of yarn but the Peacock colour has discontinued. Please advise how I can change this pattern to sleeveless waistcoat. Many thanks in advance for advice.
Alicia Jacket - Knitting Pattern for Women in MillaMia Naturally Soft Merino - Downloadable PDF | LoveCrafts

So annoying when this happens. Have you tried Ravelry? The yarn in peacock has been stashed by many member (7 pages worth of them). If you’d like to finish the pattern with sleeves, someone will surely be able to help with the yarn. Some posters even indicate a willingness to sell or trade.
https://www.ravelry.com/yarns/library/millamia-naturally-soft-merino/stashes?cf=%3F&page=7&photoless=0&search=peacock&status=

It’s a very unusual jacket with beautiful stitch pattern. This Millamia yarn is particularly lovely to work with.

Have you managed to find more yarn?

I think I’d shape the armhole a little more if it was mine. I’d quite like to remove the drop on the shoulder and have it stop on the outer edge of my shoulder. I’d measure my width shoulder to shoulder and calculate how many stitches I needed to decrease by. Then I’d do the decreases from the underarm bind off on each side.
A small rib around the armhole may look nice, or a crochet edge.

thanks for the nice suggestions. Unfortunately, I can only knit following all the instructions on the pattern. I am no good at making alternations. Will you be able to help me? The following are the instructions for the back and the front. I am knitting the 2nd size. Thank you so much in advance

The Back - Shape back neck and shoulders
1st row Patt 37(40:43:49:52:55), work 2 tog, turn and work on these sts.
2nd row P2 tog tbl, p to end.
3rd row Cast off 12(13:14:16:17:18) sts, patt to end.
Rep the last 2 rows once more.
Work 1 row.
Cast off rem 12(13:14:16:17:18) sts.
With right side facing rejoin yarn to rem sts, cast off centre
57(60:63:63:66:69) sts, work 2 tog, patt to end.
1st row Cast off 12(13:14:16:17:18) sts, patt to last 2 sts, p2 tog.
2nd row Patt to end.
Rep the last 2 rows once more.
Cast off rem 12(13:14:16:17:18) sts.

Left Front
Shape front neck
Next row Patt 43(46:49:53:56:59), turn and leave rem
18(21:21:23:23:26) sts on a holder.
Next row P2 tog tbl, patt to end.
Next row Patt to last 2 sts, skpo.
Rep the last 2 rows 4 times. 33(36:39:43:46:49) sts.
Work straight until front measures the same as back to
shoulder shaping, ending at armhole edge.
Shape shoulder
1st row Cast off 12(13:14:15:16:17) sts, patt to end.
2nd row Patt to end.
3rd row Cast off 11(12:13:15:16:17) sts, patt to end.
4th row Patt to end.
Cast off rem 10(11:12:13:14:15) sts.

You would probably need to change the shaping earlier than this, at the underarm bind off and shape from there. If you have already worked past that section it would mean undoing som of the knitting.
Or follow the shaping as given for a sort of t-shirt type style. You can see the drop shoulder on the pattern photo, and the seam where the sleeve is seamed. Do you think you’d like it like that? It is an easier modification. You could pick up around the arm hole to add a ribbing section.