To answer this question, I first need to explain an interesting quirk of the phrase 'be supposed to'.
Merriam-Webster definition: be supposed to
- to be expected to do something (They are supposed to arrive tomorrow.)
When used in the present tense, as in the example given by the Merriam-Webster dictionary, the meaning of 'be supposed to' is positive.
They are supposed to arrive tomorrow. = They are expected to arrive tomorrow.
However, when used in the past tense the meaning of 'be supposed to' becomes negative.
They were supposed to arrive tomorrow. = They were expected to arrive tomorrow, but now they are not going to arrive tomorrow.
As stated in one of the comments above, the original examples contain some logical problems, so I'll answer this two ways.
If the main point is the attendance to the concert:
I was supposed to go the concert, but it was absolutely pouring.
I went to the concert, but it was absolutely pouring.
In the first case, you intended to go to the concert but did not go because it was raining.
In the second case, you went to the concert. It is implied with the negative conjunction 'but' and following phrase that you did not enjoy the event as much as you might have because it was raining.
Alternatively, if the emphasis is the mode of transportation:
I was supposed to take the bus to the concert, but it was absolutely pouring, so I decided to go by car instead.
When I went to the concert it was absolutely pouring, so I decided to go by car.
I was supposed to take the bus to the concert, but it was absolutely pouring.
In the first case, the original plan (to take the bus), the rationale for the change (it was raining), and the final course of action (I decided to go by car) are all clearly stated. 'Instead' is not required for this sentence, but as a native speaker I find it almost impossible to say this sentence without attaching the word 'instead' at the end.
In the second case, the final course of action (so I decided to go by car) implies a change of plans, but the original plan is unknown (e.g. bus, train?).
In the third case, without the final course of action, two potential meanings are possible:
- Because it was pouring you didn't go to the concert.
- Because it was pouring you took an alternate means of transportation.
Generally this shortened sentence without the 'final course of action' would only be used if key facts such as 'the concert was cancelled' or 'you drove to the concert' was already known from earlier context.