As the fight is designed around the characters ending the campaign at level 5, you would have 2 options:
- Allow parts from level 5 to be easier than expected
- Tweak meets her to make it harder
Personally I would do a little of both. As GM, it can be easy to think that harder fight = more exciting for players, But it's not always the case. Sometimes, a few easy matches will give the players a sense of accomplishment.
For example, when players cleared Cragmaw's hideout, they probably found it difficult. If you throw them a similar fight at level 4 or 5, they will just get through. Some players may not find it interesting, but others will look back and say "wow look how strong we are".
When all the fights are difficult, it can be difficult for the party to find that it has progressed.
I did it in the past, not with the LMoP, but with other adventures. The main objective here is to compensate for the extra power you have given them. Try to analyze how difficult the meeting should have been. If you are new, you can try to execute the combat alone (using the character sheets of the PC) at the desired level. Then adjust the fight, put the characters to their current level and try again.
It may sound like a lot of work, but when I started out as a DM, I found it very useful. RC is at best a guideline. The best way to know if a match is too strong or too weak (as a young beginner) is to play it.
In conclusion, if you do not do anything with the encounters as they are written, the party will simply find the counters easier than expected by the designer. I would recommend modifying the meetings if you plan to organize a party beyond level 5.